Admin interface: use autocomplete to change responsible person of a set

Squashed commit of the following:

commit b1f4219d3ecd8ddab7493d6c407dd4b9f777adb0
Author: Michał Podlecki <m.podlecki@op.pl>
Date:   Tue Aug 26 10:46:22 2014 +0200

    Admin interface: use autocomplete to change responsible person of a set