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.
OK, that makes sense.

It's probably not in the commit history, but this actually started out in presave_adjustments - I ended up moving it because we need to prevent certain age groups from getting past the registration page (AnthroCon specifically does not sell badges to attendees under 13).

From what I could tell, presave_adjustments doesn't kick in until after the attendee has paid with Stripe. It would be confusing and potentially frustrating for someone to pay for a registration and THEN be told they can't buy a badge.

Is there an alternate way to prevent attendees in certain age groups from registering?