Overhaul Repository Handling

This changes how we handle repositories according to https://gitlab.com/fdroid/wiki/-/wikis/Repository-Management-Overhaul/Representing-apps-from-multiple-repos

It includes a non-trivial database migration and should be tested thoroughly before releasing to users.

Attention: This MR is meant to be atomic. You can not simply cherry-pick individual commits and expect things to work. There's lots of dependencies between the individual commits, e.g. on the app and DB layer.

Fixes #702 (closed), #1566 (closed), #1887 (closed), #2681 (closed)