Split participant functionality from category filter functionality so
that category filter can be used seperately if necessary - now ParticipantMixin inherits from
CategoryFilterMixin (and both moved into new module
participant/mixins.py), and participant_selector.html 'include's
category_filter.html.
Merge branch 'master' into participant_views_4
Merge pull request #66 from jogwen/participant_views_4

Participant views 4