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.
Whoops, the "not" shouldn't be there.

In general, I'm having trouble doing a conversion in the code - a lot of times where it's checking for UNDER_18, the age_group hasn't been set yet, so it throws a None error.