Add email_access_valdiated_at field to user table, populate it

and update it when users have to use their email to interact with
Notify service.

Initial population:
If user has email_auth, set last_validated_at to logged_in_at.
If user has sms_auth,  set it to created_at.

Then:
Update email_access_valdiated_at date when:
- user with email_auth logs in
- new user is created
- user resets password when logged out, meaning we send them an
email with a link they have to click to reset their password.

app/schemas.py

@@ -101,6 +101,7 @@ class UserSchema(BaseSchema):
         model = models.User
         exclude = (
             "updated_at",
+            "email_access_validated_at",
             "created_at",
             "user_to_service",
             "user_to_organisation",