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/models.py

@@ -114,6 +114,7 @@ class User(db.Model):
     platform_admin = db.Column(db.Boolean, nullable=False, default=False)
     current_session_id = db.Column(UUID(as_uuid=True), nullable=True)
     auth_type = db.Column(db.String, db.ForeignKey('auth_type.name'), index=True, nullable=False, default=SMS_AUTH_TYPE)
+    email_access_validated_at = db.Column(db.DateTime, index=False, unique=False, nullable=False)
 
     # either email auth or a mobile number must be provided
     CheckConstraint("auth_type = 'email_auth' or mobile_number is not null")
@@ -162,6 +163,7 @@ class User(db.Model):
             'auth_type': self.auth_type,
             'current_session_id': self.current_session_id,
             'failed_login_count': self.failed_login_count,
+            'email_access_validated_at': self.email_access_validated_at,
             'logged_in_at': self.logged_in_at.strftime(DATETIME_FORMAT) if self.logged_in_at else None,
             'mobile_number': self.mobile_number,
             'organisations': [x.id for x in self.organisations if x.active],