This commit replaces the previous `StringField` used for collecting mobile
phone numbers with the `UKMobileNumber` field.
This means changing a few of the preexisting tests to have more realistic mobile
numbers so that they still pass.
Fixed the is_active() method on the Users model, if the user was pending they would come back as active, allowing a user to sign in before being active.
There is still a problem with the validate_sms_code and validate_email_code method.