Present tracks' sheet music & MIDIs on album page

[towerofnix]

Following up #446, it would be nice if the album's sections for sheet music and MIDIs could automatically present you with all the sheet music and MIDIs across the album's tracks, not just the entries placed on the album itself. (That is, we'd show an interface combining them if either or both the album itself and its tracks have any additional files of the given category.)

We're basically thinking of something inspired by All Sheet Music, just filtered down for the current album only. We probably want this to be that the full list can be toggled open/closed at once, with no toggles for the individual tracks, so you either get everything at once or nothing (the default).

Tracks also support additional files (apparently), and these should be supported in this interface, too.