MBS-6353: Show more information on release group merge edits

Now uses a table to display the release group name, the artist credit,
the release group types, the first release date year, and the amount
of releases inside the release group.

I've had to slightly mame Data::Edit's loading, by introducing a
'pseudo-model' - *Meta. By using this model, it will actually call
the load_meta method. It's a bit of a gross hack, but does allow
us to access _meta information.
Merge branch 'mbs-6353' into beta