Rework media group handling

As per a new discussion, we need to change the way we handle media groups.

Notably, a media must be in a group at all time. Those "automatic" groups are meant to allow the UI to list all media through the media groups listing.

Those groups:

• Are the only candidates when trying to regroup a media with other similarly named media (technically, the media contained is those groups are the only candidates)
• Can't be renamed
• Can't be deleted (since they would be recreated immediately)
• Can be ordered just like any other group

For all groups:

• It's still possible to add/remove any media to any group
• When a media is removed, it should be added to its "single media" group
• An empty group should be removed automatically