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.
This was actually discussed/determined in AnthroCon's slack chat, which you should totally accept the invite to when you aren't metaphorically buried under work and snot. :)