Set session rather than stub context so we are free to refactor

also remove httmocks - they are causing tests to hang so I
am going to be explicit outside of status and patch things directly
Tests for new signed in method
Add a signed in method
Refactor - signed in separated out of template context

template context is a better name. Stuff for all templates (base and
nav) is in base template context. Routes build there own context. If
signed in they have access to the admin client.
Add test for handling 403s

remove unnecessary patch
Add code to handle 403s
Move environment_dict to base_template_context, remove method

as per robs comment in pull request
Merge pull request #26 from alphagov/catch-expired-token-403

Catch expired token 403