Ensure that the session is logged out server side, not just client side.

Anytime a user clicks "sign out" we should be signing them out server side as well. This can be accomplished by setting the Users.current_session_id = null.
I found that the method User.logged_in_elsewhere doesn't need to check if the current_session_id is None. The current_session_ids in the cookie and db (redis or postgres) then the user should be forced to log in again.

tests/app/main/views/test_sign_in.py

@@ -60,8 +60,7 @@ def test_doesnt_redirect_to_sign_in_if_no_session_info(
 
 
 @pytest.mark.parametrize('db_sess_id, cookie_sess_id', [
-    pytest.param(None, None, marks=pytest.mark.xfail),  # OK - not used notify since browser signout was implemented
-
+    (None, None),
     (uuid.UUID(int=1), None),  # BAD - has used other browsers before but this is a brand new browser with no cookie
     (uuid.UUID(int=1), uuid.UUID(int=2)),  # BAD - this person has just signed in on a different browser
 ])

tests/app/main/views/test_sign_out.py

@@ -4,9 +4,9 @@ from tests.conftest import SERVICE_ONE_ID
 
 
 def test_render_sign_out_redirects_to_sign_in(
-    client
+    logged_in_client
 ):
-    response = client.get(
+    response = logged_in_client.get(
         url_for('main.sign_out'))
     assert response.status_code == 302
     assert response.location == url_for(