- Add unit test for when case when the cookie doesn't match the db.

- Move code into User.signout method to further encapsulate the code.

app/models/user.py

@@ -1,5 +1,5 @@
 from flask import abort, current_app, request, session
-from flask_login import AnonymousUserMixin, UserMixin, login_user
+from flask_login import AnonymousUserMixin, UserMixin, login_user, logout_user
 from notifications_python_client.errors import HTTPError
 from notifications_utils.timezones import utc_string_to_aware_gmt_datetime
 from werkzeug.utils import cached_property
@@ -155,6 +155,8 @@ class User(JSONModel, UserMixin):
 
     def sign_out(self):
         # Update the db so the server also knows the user is logged out.
+        session.clear()
+        logout_user()
         return self.update(current_session_id=None)
 
     @property