Allow only main organisers to accept participants
Work on SCC styles
Experiment with participant views
Work on filtering form
Work on filtering helper function
Add custom participant filters

Add ad hoc filters to select participants with talk proposals and to filter participants by the number of conferences to attend.
Remove 'participants_with_talk_proposals' page

With the new custom participant filter, this organisers' page is no longer needed.
Combine 2 filtering forms into one, change helper