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

tests/app/main/views/test_accept_invite.py

@@ -80,7 +80,7 @@ def test_if_existing_user_accepts_twice_they_redirect_to_sign_in(
         page.select('main p')[0].text.strip(),
     ) == (
         'You need to sign in again',
-        'We sign you out if you haven’t used Notify for a while.',
+        'We signed you out because you haven’t used Notify for a while.',
     )
 
 
@@ -106,7 +106,7 @@ def test_existing_user_of_service_get_redirected_to_signin(
         page.select('main p')[0].text.strip(),
     ) == (
         'You need to sign in again',
-        'We sign you out if you haven’t used Notify for a while.',
+        'We signed you out because you haven’t used Notify for a while.',
     )
     assert mock_accept_invite.call_count == 1
 
@@ -141,7 +141,7 @@ def test_existing_signed_out_user_accept_invite_redirects_to_sign_in(
         page.select('main p')[0].text.strip(),
     ) == (
         'You need to sign in again',
-        'We sign you out if you haven’t used Notify for a while.',
+        'We signed you out because you haven’t used Notify for a while.',
     )