Improve handling of deleted tracks

following this thread : https://forum.funkwhale.audio/d/66-why-doesnt-this-track-play-for-me

We could make some change to the UI to handle deleted tracks.

Hide track while browsing but show it disabled in playlist, favoured and listened.

In the track page :

• Delete or grey the play button
• Indicate this version of the track isn't present on the network anymore.
• Create a link to another version of the same track ?

In the album page and artist page, deleted track appear as readable :§

• Delete or grey the play buttons

In the playlist and radio page ?

I supposed this can be done using the is_playable attribute.