Implement category filtering.
Avoid repetition for common functionality by putting it in a mixin
class.
Implemented a new RedirectView which redirects category filter requests
to the detail page of the first matching participant, or to the landing
page if none, preserving the selected category in both cases.
Only show approved Participants on website.
Ensure that content following the left-floated participant selector bar
clears it, and add white padding around logos.
Provide default manager for Participant as well otherwise we only see
approved ones everywhere (including admin screens).
Merge branch 'master' into participant_views_2
Merge pull request #63 from jogwen/participant_views_2

Participant views - filter working, hiding unapproved Participants, more to do ...