Event collaborators model change

https://github.com/e-Luminate/eluminate_web/issues/127
Each Event can now have a set of collaborating Participants as well as the owner Participant. 
A south migration is included to fix any existing database (ie to add the table, so won't create any relationships). 
All screens listing Events now display Events collaborated upon by relevant, approved Participants as well as those Events owned by relevant, approved Participants. New Participant model method approved_collaboration_events helps with this.
All screens showing Event summary/details now display approved collaborating Participants as well as the owner Participant - new Event model method approved_collaborators helps with this. Also made more things clickable in Event summaries (to click through to Event detail).
Event.save() and a signal handler on Event.collaborators M2M field prevent an Event owner becoming a collaborator, and vice-versa.