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.
Oh yes, I see what you mean.

I'm actually going to work around this by checking for the birthdate value, and returning "None" if it doesn't exist, because there will be events who want the birthdate column to always be None.