Add tests for media set highlights, cover and content display mode
Admin interface: delete groups, merge inst. groups

Admin interface:
- add delete button in groups list
- ability to merge institutional groups

Squashed commit of the following:

commit c094cf96363080950283a10cf8dc15eaac4e8ef0
Author: Michał Podlecki <m.podlecki@op.pl>
Date:   Wed Nov 12 13:05:25 2014 +0100

    Admin interface:
    - add delete button in groups list
    - ability to merge institutional groups