Birth date

Original pull-request closed, and replaced with this one as per Loek's instructions.
See: https://github.com/onepercentclub/onepercentclub-site/pull/774

Due to an issue with localized <-> UTC datetimes, the birtdate was incorrectly sent to the server: 01/01/1980 becomes 31/12/1979.

Fixed by adding a 'birthdate' transform taking the date from the localized date object. The time portion is irrelevant, as the Django field is a DateField.