Fix UX & performance issues on backend participant form

I've used the new select2/entityReference widget to fix some longstanding problems with the backend participant registration form, namely:
Every single event in the db was being added to the DOM twice (sometimes 4 times) causing major scaling headaches.
The "show past 3 months" and "show all" buttons were not great UX - breaks the rule of "don't make me think"
Some unnecessary ajax calls e.g. to fetch default role when selecting event - it's better to just return this data as part of the autocomplete results.