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.

app/main/encryption.py

@@ -6,4 +6,5 @@ def hashpw(password):
 
 
 def checkpw(password, hashed_password):
+    # If salt is invalid throws a 500 should add try/catch here
     return check_password_hash(hashed_password, password)