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.
The issue is that the testdata doesn't have ``birthdate`` set.  A temporary workaround in the Anthrocon Uber would be that in ``import_uber_test_data`` in the ``import_attendees`` function you could do something like ``attendee['birthdate'] = <random birthdate, or maybe a birthdate based on the age_group>``.  Then you wouldn't run into the issue where ``birthdate`` is None.