communities: ranker periodic task

* Introduces periodic task which can be run to
  rank communities.

* Adds admin interface for communities module.

* Adds search box inside communities module
  and pagination.

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
communities: query improvements and PEP8 fixes

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>