Include "Superuser" in group names for admin accounts listing.

This is to make it easier from the list page to determine who's a
superuser without having to add another column (since while superuser
isn't a real group, it acts as a pseudo group).
Fix admin form not saving unchecked group memberships.

For some reason, ember-data was doing something weird with the
association setup, and always sending the submission form the old group
values once one group was selected. Since we're not really using the
association on the ember-data side of things, removing it seems to do
the trick.
Try to fix a capybara timing issue.