Mark whether an item belongs to a collection

[lydavid]

this will allow me to "complete" artists by adding all of their release groups to a collection
and it gives me feedback that I've added the entity to a collection already

highly requested feature in MB: https://tickets.metabrainz.org/browse/MBS-3491

impl

check for it in collection_entity

how do other apps show this?

• Spotify uses a green checkmark
  • clicking on it will bring up the "Add to playlist" screen that allows user to add the item to other playlists
• Wikipedia has a solid bookmark when you visit a page you've saved
• others uses a star