Remove external media from groups

This is a followup for !422 (merged) / #344 (closed)

We currently filter out external media when returning media groups members. Ideally, we shouldn't have to do that and should remove the external media from the groups altogether.

The drawback from doing this is that if you remove an indexed folder by mistake, you won't be able to cancel that removal so this scenario:

• A user adds a folder to the indexed locations
• They spend an hour classifying all their media in groups
• User removes the indexed folder by mistake
• All groups are lost
• Angry user says grrrr

couldn't be recovered from.

We need to discuss if we're OK removing all groups from the database, or if we can live with the (hopefully small) cost of filtering every time