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

requirements.txt

@@ -7,7 +7,7 @@ ago==0.0.93
 humanize==1.0.0
 Flask==1.1.1
 Flask-WTF==0.14.3
-Flask-Login==0.4.1
+Flask-Login==0.5.0
 
 blinker==1.4
 pyexcel==0.5.15
@@ -30,10 +30,10 @@ git+https://github.com/alphagov/notifications-utils.git@36.6.2#egg=notifications
 git+https://github.com/alphagov/govuk-frontend-jinja.git@v0.5.1-alpha#egg=govuk-frontend-jinja==0.5.1-alpha
 
 ## The following requirements were added by pip freeze:
-awscli==1.18.13
+awscli==1.18.15
 bleach==3.1.1
 boto3==1.10.38
-botocore==1.15.13
+botocore==1.15.15
 certifi==2019.11.28
 chardet==3.0.4
 Click==7.0