fix registration_id uniqueness/overwriting
fix registration_id uniqueness/overwriting

Line 85 would throw an error if 'id' was not included in the PATCH request (which it might not be)
Also, lines 91-94 would probably throw an error if 'registration_id' wasn't in the request (which it might not be)

Removed UniqueValidationMixin since it has diverged from OverwriteValidation and we don't use it anymore
CustomerSupport/django-push-notifications