Add image upload methods

We use the "manual" way as it was before, since there were some problems
making this work and most of the errors were happening during the exercise
creation itself.

lib/models/exercises/exercise_submission.freezed.dart

@@ -50,7 +50,6 @@ abstract mixin class $ExerciseAliasSubmissionApiCopyWith<$Res> {
   factory $ExerciseAliasSubmissionApiCopyWith(
           ExerciseAliasSubmissionApi value, $Res Function(ExerciseAliasSubmissionApi) _then) =
       _$ExerciseAliasSubmissionApiCopyWithImpl;
-
   @useResult
   $Res call({String alias});
 }
@@ -236,7 +235,6 @@ extension ExerciseAliasSubmissionApiPatterns on ExerciseAliasSubmissionApi {
 @JsonSerializable()
 class _ExerciseAliasSubmissionApi implements ExerciseAliasSubmissionApi {
   const _ExerciseAliasSubmissionApi({required this.alias});
-
   factory _ExerciseAliasSubmissionApi.fromJson(Map<String, dynamic> json) =>
       _$ExerciseAliasSubmissionApiFromJson(json);
 
@@ -282,7 +280,6 @@ abstract mixin class _$ExerciseAliasSubmissionApiCopyWith<$Res>
   factory _$ExerciseAliasSubmissionApiCopyWith(
           _ExerciseAliasSubmissionApi value, $Res Function(_ExerciseAliasSubmissionApi) _then) =
       __$ExerciseAliasSubmissionApiCopyWithImpl;
-
   @override
   @useResult
   $Res call({String alias});
@@ -350,7 +347,6 @@ abstract mixin class $ExerciseCommentSubmissionApiCopyWith<$Res> {
   factory $ExerciseCommentSubmissionApiCopyWith(
           ExerciseCommentSubmissionApi value, $Res Function(ExerciseCommentSubmissionApi) _then) =
       _$ExerciseCommentSubmissionApiCopyWithImpl;
-
   @useResult
   $Res call({String alias});
 }
@@ -536,7 +532,6 @@ extension ExerciseCommentSubmissionApiPatterns on ExerciseCommentSubmissionApi {
 @JsonSerializable()
 class _ExerciseCommentSubmissionApi implements ExerciseCommentSubmissionApi {
   const _ExerciseCommentSubmissionApi({required this.alias});
-
   factory _ExerciseCommentSubmissionApi.fromJson(Map<String, dynamic> json) =>
       _$ExerciseCommentSubmissionApiFromJson(json);
 
@@ -582,7 +577,6 @@ abstract mixin class _$ExerciseCommentSubmissionApiCopyWith<$Res>
   factory _$ExerciseCommentSubmissionApiCopyWith(
           _ExerciseCommentSubmissionApi value, $Res Function(_ExerciseCommentSubmissionApi) _then) =
       __$ExerciseCommentSubmissionApiCopyWithImpl;
-
   @override
   @useResult
   $Res call({String alias});
@@ -615,16 +609,11 @@ class __$ExerciseCommentSubmissionApiCopyWithImpl<$Res>
 /// @nodoc
 mixin _$ExerciseTranslationSubmissionApi {
   String get name;
-
   String get description;
-
   int get language;
-
   @JsonKey(name: 'license_author')
   String get author;
-
   List<ExerciseAliasSubmissionApi> get aliases;
-
   List<ExerciseCommentSubmissionApi> get comments;
 
   /// Create a copy of ExerciseTranslationSubmissionApi
@@ -667,7 +656,6 @@ abstract mixin class $ExerciseTranslationSubmissionApiCopyWith<$Res> {
   factory $ExerciseTranslationSubmissionApiCopyWith(ExerciseTranslationSubmissionApi value,
           $Res Function(ExerciseTranslationSubmissionApi) _then) =
       _$ExerciseTranslationSubmissionApiCopyWithImpl;
-
   @useResult
   $Res call(
       {String name,
@@ -916,7 +904,6 @@ class _ExerciseTranslationSubmissionApi implements ExerciseTranslationSubmission
       final List<ExerciseCommentSubmissionApi> comments = const []})
       : _aliases = aliases,
         _comments = comments;
-
   factory _ExerciseTranslationSubmissionApi.fromJson(Map<String, dynamic> json) =>
       _$ExerciseTranslationSubmissionApiFromJson(json);
 
@@ -930,7 +917,6 @@ class _ExerciseTranslationSubmissionApi implements ExerciseTranslationSubmission
   @JsonKey(name: 'license_author')
   final String author;
   final List<ExerciseAliasSubmissionApi> _aliases;
-
   @override
   @JsonKey()
   List<ExerciseAliasSubmissionApi> get aliases {
@@ -940,7 +926,6 @@ class _ExerciseTranslationSubmissionApi implements ExerciseTranslationSubmission
   }
 
   final List<ExerciseCommentSubmissionApi> _comments;
-
   @override
   @JsonKey()
   List<ExerciseCommentSubmissionApi> get comments {
@@ -1001,7 +986,6 @@ abstract mixin class _$ExerciseTranslationSubmissionApiCopyWith<$Res>
   factory _$ExerciseTranslationSubmissionApiCopyWith(_ExerciseTranslationSubmissionApi value,
           $Res Function(_ExerciseTranslationSubmissionApi) _then) =
       __$ExerciseTranslationSubmissionApiCopyWithImpl;
-
   @override
   @useResult
   $Res call(
@@ -1065,20 +1049,14 @@ class __$ExerciseTranslationSubmissionApiCopyWithImpl<$Res>
 /// @nodoc
 mixin _$ExerciseSubmissionApi {
   int get category;
-
   List<int> get muscles;
-
   @JsonKey(name: 'muscles_secondary')
   List<int> get musclesSecondary;
-
   List<int> get equipment;
-
   @JsonKey(name: 'license_author')
   String get author;
-
   @JsonKey(includeToJson: false)
   int? get variation;
-
   List<ExerciseTranslationSubmissionApi> get translations;
 
   /// Create a copy of ExerciseSubmissionApi
@@ -1129,7 +1107,6 @@ abstract mixin class $ExerciseSubmissionApiCopyWith<$Res> {
   factory $ExerciseSubmissionApiCopyWith(
           ExerciseSubmissionApi value, $Res Function(ExerciseSubmissionApi) _then) =
       _$ExerciseSubmissionApiCopyWithImpl;
-
   @useResult
   $Res call(
       {int category,
@@ -1389,14 +1366,12 @@ class _ExerciseSubmissionApi implements ExerciseSubmissionApi {
         _musclesSecondary = musclesSecondary,
         _equipment = equipment,
         _translations = translations;
-
   factory _ExerciseSubmissionApi.fromJson(Map<String, dynamic> json) =>
       _$ExerciseSubmissionApiFromJson(json);
 
   @override
   final int category;
   final List<int> _muscles;
-
   @override
   List<int> get muscles {
     if (_muscles is EqualUnmodifiableListView) return _muscles;
@@ -1405,7 +1380,6 @@ class _ExerciseSubmissionApi implements ExerciseSubmissionApi {
   }
 
   final List<int> _musclesSecondary;
-
   @override
   @JsonKey(name: 'muscles_secondary')
   List<int> get musclesSecondary {
@@ -1415,7 +1389,6 @@ class _ExerciseSubmissionApi implements ExerciseSubmissionApi {
   }
 
   final List<int> _equipment;
-
   @override
   List<int> get equipment {
     if (_equipment is EqualUnmodifiableListView) return _equipment;
@@ -1430,7 +1403,6 @@ class _ExerciseSubmissionApi implements ExerciseSubmissionApi {
   @JsonKey(includeToJson: false)
   final int? variation;
   final List<ExerciseTranslationSubmissionApi> _translations;
-
   @override
   List<ExerciseTranslationSubmissionApi> get translations {
     if (_translations is EqualUnmodifiableListView) return _translations;
@@ -1491,7 +1463,6 @@ abstract mixin class _$ExerciseSubmissionApiCopyWith<$Res>
   factory _$ExerciseSubmissionApiCopyWith(
           _ExerciseSubmissionApi value, $Res Function(_ExerciseSubmissionApi) _then) =
       __$ExerciseSubmissionApiCopyWithImpl;
-
   @override
   @useResult
   $Res call(