Adding riders to carpools

Adds better 'add rider to carpool' functionality. I'm submitting this now for demonstration at the meeting tomorrow, but here are a few things that I am still working on

* Email updates
* authorization, checking people can actually add a ride to a carpool
* checking to see if a rider is already part of any carpool going to an event
* ride requests
* checking carpool vacancy when adding riders
* confirming riders

we need to think about the best way to do the 'I need a ride' feature because without a carpool that rider has no link to an event. I'll continue working on the stuff above.