feat(subsonic): support public playlists

When multiple people share the same instance, they might want to share
their playlists between them.

This allows people to mark playlists as public, and to listen to public
playlists from other people. Listeners will also know who owns the
playlist, to help avoid confusion and make this feature a bit nicer.

Subsonic restrict updating playlists only to owners, this honors that
behavior, but adding flexibility could be achieved easily.

server/db/model.go

@@ -247,6 +247,7 @@ type Playlist struct {
 	Comment    string
 	TrackCount int
 	Items      string
+	IsPublic   bool `sql:"default: null"`
 }
 
 func (p *Playlist) GetItems() []int {