communities: featured community UI problems fixup

* Fixes the `Curate` button link and the translated string
  for the empty collection.

Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Signed-off-by: Adrian Pawel Baran <adrian.pawel.baran@cern.ch>
alerts: PEP8/257 improvements in models

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
search: UserQuery relationship addition

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