Add an optional "buy this music" link

[lamarqua]

It would be awesome if on top of the spotify / youtube / streaming service link, there was a way for submitters to put a link to a page where you can buy the music (like bandcamp), although arguably this is a niche ask

[lamarqua]

Suggestion for UI:

• add a regular field when adding / editing the track
• add a right aligned column in the track list that contains the link with a $ icon if the field exists