Mark tracks as instrumental

[euricaeris]

Seems like it'd be useful for figuring out where best to slot a song into a show, knowing ahead of time that it has no vocals.

This could also be an admin-mode marker, not sure how best to display it.

[theshillito]

I think it would be good to have it be public info. However, I wonder if this would be under the tagging system suggested in #148 rather than its own thing