Enable exact birthdates

Adds support for collecting the exact birthdate from attendees. Fixes #586.

The database on live/staging servers will need the "birthdate" column added to the Attendees table and the AgeGroups table added before this code can be used on them.
Unfortunate, but sensible.  Again, this is definitely a change that should be a lot easier than it is, but our code is probably about a year away from being in good enough shape to make this kind of thing as easy as it should be :(

It sounds like Anthrocon is doing a lot of really cool work (e.g. bootstrap) that we'll definitely want, and we can always just cherrypick those parts for now.  I know Dom wants to start tracking exact birthdates for M14, so we can circle back around to this when people have time.  If there's any kind of consensus that we'd really like to be able to do pull requests, we can circle back around to this sooner.  Sorry again for not being as available to help work through this kind of thing :(