upgrade flask_login to 0.5.0

flask_login sets a bunch of variables in the session object. We only use
one of them, `user_id`. We set that to the user id from the database,
and refer to it all over the place.

However, in flask_login 0.5.0 they prefix this with an underscore to
prevent people accidentally overwriting it etc. So when a user logs in
we need to make sure that we set user_id manually so we can still use
it.

flask_login sets a bunch of variables on the `flask.session` object.
However, this session object isn't the one that gets passed in to the
request context by flask - that one can only be modified outside of
requests from within the session_transaction context manager (see [1]).
So, flask_login populates the normal session and then we need to copy
all of those values across.

We didn't need to do this previously because we already set the
`user_id` value on line 20 of tests/__init__.py, but now that
flask_login is looking for `_user_id` instead we need to do this
properly.

[1] https://flask.palletsprojects.com/en/1.1.x/testing/#accessing-and-modifying-sessions

app/models/user.py

@@ -139,6 +139,7 @@ class User(JSONModel, UserMixin):
 
     def login(self):
         login_user(self)
+        session['user_id'] = self.id
 
     def sign_in(self):