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.
It does make sense, but the "can_register" value is stored in the age group itself for easy configuration - so wouldn't the model_checks validation code still have to calculate the age group that matches the birthdate?