check users' session id.

when a user enters their 2FA code, the API will store a random UUID
against them in the database - this code is then stored on the cookie
on the front end.

At the beginning of each authenticated request, we do the following
steps:
  * Retrieve the user's cookie, and get the user_id from it
  * Request that user's details from the database
  * populate current_user with the DB model
  * run the login_required decorator, which calls
    current_user.is_authenticated

is_authenticated now also checks that the database model matches the
cookie for session_id. The potential states and meanings are as follows:

 database | cookie | meaning
----------+--------+---------
 None     | None   | New user, or system just been deployed.
          |        | Redirect to start page.
----------+--------+---------
 'abc'    | None   | New browser (or cleared cookies). Redirect to
          |        | start page.
----------+--------+---------
 None     | 'abc'  | Invalid state (cookie is set from user obj, so
          |        | would only happen if DB is cleared)
----------+--------+---------
 'abc'    | 'abc'  | Same browser. Business as usual
----------+--------+---------
 'abc'    | 'def'  | Different browser in cookie - db has been changed
          |        | since then. Redirect to start

app/notify_client/user_api_client.py

@@ -1,3 +1,6 @@
+import uuid
+
+from flask import session
 from notifications_python_client.errors import HTTPError
 
 from app.notify_client import NotifyAdminAPIClient