This commit:
- replaces links that look like buttons with forms and submit buttons
- splits the view code for SMS into its own file
- moves the routing into the Python by adding handling for `post` requests
- uses Flask’s `url_for` to generate URLs, rather than hard coding them (so that
it’s easier to change the URLs)
- chages the URLs for sending text messages
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.
