Simplify ValidateEmailMixin

* Remove unused check for missing email field
* Increase test coverage
Increase coverage of GetToken.delete to 100%
Remove untested code from VerifyUserAdmin

* Assume the is_active field exists in the fieldsets
* If different behaviour is required VerifyUserAdmin can be subclassed
Add test for UserCreationForm.save

* Restrict UserCreationForm fields to email
Add missing tests for CaseInsensitiveEmailBackend