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.
Do not save the playback state when sanitizing.
After some thought, there is no situation where re-saving the playback
state is desirable. A previously saved state will be consistent with
a sanitized state, and there is no need to save when the service is
active. Thus, save on speed and reduce insane race conditions by just
sanitizing the current runtime state and not saving at all.
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.
