Enrollment Flow Client Side Form Validation

The forms generated for enrollment flows should perform client side data validation wherever possible. There are two approaches here:

1 - Define validation routines in javascript and add configuration support so that the appropriate routine is called when a form field is edited.
2 - Set up an API call that allows field data to be validated using the cake validation rules.

#2 is probably preferable, since #1 would require the javascript routines and the cake validation routines to be kept in sync. However, then the checks are not purely client side.

After this is implemented, consider new tickets for adding the same functionality to other forms.