Make rails eager load categories and events for attendees, also add futu...

...re attendee scope.
Not really important, but I had this on my conscience! While working on an unrelated project, I learned about eager loading with rails and instantly remembered how the admin attendees page did tons of queries to 2 tables to get its data. There can be better solutions, but the proposed was simple so I did it. Also adds a 'future_attendees' scope that selects only those who are attending an event that hasn't passed. It might be useful if used in a separate page. I have commented out the call to it.