Rewrite get_age_group_date

Rewrite of the get_age_group_date to be based on the database min_age
and max_age.
Adds birthdate/age-group conversion and reg check

model_checks.py gets a big ugly function that takes a mm-dd-yy formatted
date and finds its associated age group.

The method for doing so is non-ideal - the age_group options must have
descriptions matching the database entries - but it will be usable
without any data loss.

This function also throws an error if the age group's can_register value
is False.
Front-end changes for birthdate

Adds the front-end input fields for birthdate, including correct
formatting.

Also adds the jQueryUI datepicker class.
Make birthdate unrestricted
Fixes age_group

Fixes a typo that caused age_group to have no default.