Move the music cache to the app storage.
This is actually long-term data, so it makes more sense to do app
storage where it's less likely to get mangled.
Update the contribution information and templates.
The contribution information and templates were growing a bit stale,
given that they haven't gotten a refresh since ~1.3.0. This commit
reworks them to be more thorough and straightfoward.
Make it so that PlaybackStateDatabase stores the item data for PlaybackState/QueueItems as strings instead of ids, as ids are too volitaile if the music library changes.
