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.
The codes are hashed and saved to the db.
The code is marked as used once a valid code is submitted.
The code is valid for 1 hour.
The codes are no longer saved to the session.
The post register endpoint will send a random 5 digit code via sms and another via email.
If either code fails to send, the user will not be created and the person can register again.
The codes are saved to the session cookie, and expire in 1 hour.
Another iteration of this story will save the codes to a database.
Includes validation for gov.uk email address, mobile number with +44, password at least 10 char.
Form validation errors will be added to template in a later story.
User is created when form validates.
