Store high-level settings (such as federation or auth-related ones) in database (#186)

Changelog
^^^^^^^^^
Due to the work done in #186, the following environment variables have been
deprecated:

- FEDERATION_ENABLED
- FEDERATION_COLLECTION_PAGE_SIZE
- FEDERATION_MUSIC_NEEDS_APPROVAL
- FEDERATION_ACTOR_FETCH_DELAY
- PLAYLISTS_MAX_TRACKS
- API_AUTHENTICATION_REQUIRED

Configuration for this settings has been moved to database, as it will provide
a better user-experience, by allowing you to edit these values on-the-fly,
without restarting Funkwhale processes.

You can leave those environment variables in your .env file for now, as the
values will be used to populate the database entries. We'll make a proper
announcement when the variables won't be used anymore.

Please browse https://docs.funkwhale.audio/configuration.html#instance-settings
for more information about instance configuration using the web interface.