Create Profile and sync_leads: fix bug 1012904

Move choices from UserRegistrationLeadForm to Profile model
Create or update Profile in UserRegistrationLeadForm.save()
Use Rest API and sync updates to salesforce leads
At the model level no, unfortunately: https://github.com/django/django/blob/master/django/contrib/auth/models.py#L390
We do have form validation that prevents this in most cases (https://github.com/mozilla/fxoss/blob/master/registration_salesforce/forms.py#L303-L311) but that wouldn't stop someone with admin privileges from accidentally creating a duplicate email.