Add redis session support

Implementation taken from here - http://flask.pocoo.org/snippets/75/
Reauth controller

Reauth controller is triggered by a push from sign-on-o-tron. After
validating the request, all sessions associated with a user are deleted.
Set redis session expiry time to 2 hours

This is to match signon expiry time to minimise the cases where a signon
user session has expired, but the admin app user session has not
expired. This leads to 403s when we request data from stagecraft, which
will be caught, but the session length being a similar time will avoid
this in most cases.
Merge pull request #20 from alphagov/reauth_wip

Add redis sessions and reauth to admin app