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.
I added this try/except so that UTCDateTime columns can be populated with just dates. Though it may be better just to go ahead and make the birthdate column a plain Date column.