Change how the client handles multiple repos

As part of the FFDW-DVD work, we are currently looking into improving how the client represents apps from multiple repositories. A detailed design document is available for review.

TL;dr:

• new repos don't override info from old repos such as f-droid.org anymore
• users get shown when an app is available from more than one repo and can choose their preferred repo
• previously fixed and implicit repo priority can be adjusted
• updates and new installs are only considered from the repo currently responsible for the app, not from all repos as currently