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.
Yeah, if you're not actually storing a time, it would definitely be better for this to be a date.  (Which doesn't have a timezone associated with it, etc.)