Playlist playback for audio/video artifacts

[copernicus-mogley]

• [x] Make player continue to next item in main playlist automatically
• [ ] 'Play All' button for artifacts with paid content

... Can we preload the next track while one is playing?

[copernicus-mogley]

Don't really need to preload next track, it's fast enough as is. Will reconsider this when building overlay player in next version.