Implement funkwhale scrobbling

Funkwhale provide API endpoints which allow the player to report which song was played. We should bring this to the mopidy-plugin.

I want to work on this, but I do have some questions.

1. I am not sure, when to report the song was played. After playback starts? Or after the hole song is played? Or simply when it stops?
2. It seems we need to inherit from mopidy.core.CoreListener. Is it save to do this in the FunkwhaleBackend or should we implement an additional FunkwhaleFrontend for this?

There is already an scrobbler for last fm: https://github.com/mopidy/mopidy-scrobbler/blob/master/mopidy_scrobbler/frontend.py

Otter uses /api/v1/history/listenings/ for this, which does not seem to be documented somehow.