108536366: Implement register flow

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.

tests/conftest.py

@@ -1,4 +1,5 @@
 import pytest
+from _pytest.monkeypatch import monkeypatch
 from sqlalchemy.schema import MetaData, DropConstraint
 
 from app import create_app, db