Record number of audio plays

[zefir-git]

Record how many times each user (from #6) has played a given track. This feature should have an "off" switch in the config (either global or per-user).

On the front-end, this will be used to sort the user's tracks, albums, artists, etc. based on number of plays on the home page (the Library) page. It could also be used to show the user their stats.

This could either be implemented server-side, e.g. tracking how much times the audio of a track was sent or the front-end client could send a request to the server.

IMO the latter would be more reliable if the user is skipping through tracks without actually listening to them. The client could send a request to the API whenever a track has finished. The client could send fractional numbers, indicating the fraction of the track that was actually played.