We don't need to package the result for flathub anymore as this happens on
https://github.com/flathub/de.wger.flutter on flathub's infrastructure. We just
upload the linux builds to the github releases.
This workflow is not manually triggered and can create automatically the appropriate
tag. The build number is not increase to the next multiple of ten, to stay in sync with
the iOS releases, which seem to cause more trouble and often need reuploads.
The individual steps have been moved out to their own files, for better readability.
We also now build the app for all supported platforms.
