Skip tracks which are not streamable

Sometimes we would have a Deezer track which doesn't have a valid
streaming URL. Playback would stop at this point and would not resume
even after pressing FWD. 

Do not try to play track if we don't get a URL. Skip to the next track
instead.
Don't hammer our backend servers when an invalid Deezer track is queued
up. This would have caused repeated metadata requests every second.
Merge branch 'public/7.7' into public/7.8
Make use of internal image proxy optional. Only TuneIn would always use
it for now.

Add preference to Performance tab in settings web UI.
Merge branch 'public/7.8' into public/feature/imageproxy