subjectadmin.rest.assignment: Added shorthelp to test.
subjectadmin.rest.assignment: Added number_of_groups_where_is_examiner.

Needed by #364.
subjectadmin...AssignmentOverview: Removed beta for approved-previously.
subjectadmin...AssignmentOverview: Added examiner box.

Shows if you are examiner, with link to the UI if published, and a
details-expander with information about examiners.

Fixes #364.