Disable sessions/don't set a session cookie

As I understand it, our frontend apps don't use sessions and, except for
Licensing, they are stripped off requests and responses by our Varnish
configuration in puppet.

So this should make dev more like production and avoid accidentally introducing
a dependency on sessions.
Merge pull request #121 from alphagov/disable_sessions_and_session_cookie

Disable sessions/don't set a session cookie