Admin interface: context improvements

Squashed commit of the following:

commit 30d2e50026544bdb52873387ab2a729a9998dd56
Author: Michał Podlecki <m.podlecki@op.pl>
Date:   Fri Aug 8 10:18:37 2014 +0200

    Admin interface: show context's related media sets on a separate page

commit eb9ea2d9bfecc15f77048db61365ca306699e0e7
Author: Michał Podlecki <m.podlecki@op.pl>
Date:   Fri Aug 8 11:54:08 2014 +0200

    Admin interface: ability to edit contexts's description