Add an option, when merging releases, to merge the releases groups with it (i.e. just a checkbox).
The checkbox should not show up if the release groups are already the same, otherwise it would be confusing, and I think it could be checked by default.

Rationale: I'm seeing quite a few edits merging releases without merging the release group, which result in MBID loss, Type loss, or empty release groups kept open by relationships.