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.
We're already halfway there with age_group_from_birthdate, right? This sounds like a pretty good solution. Does that function need to be taken out of the Session Mixin class?