Bugfix: CSRF check rejecting reauth

This makes the reauth view exempt from the CSRF check that Django adds
by default. This also updates the tests so that this is actually picked
up (Django switches the check off by default).

https://www.pivotaltracker.com/story/show/77119220
https://docs.djangoproject.com/en/dev/ref/contrib/csrf/#testing
Merge pull request #186 from alphagov/csrf-reauth

Bugfix: CSRF check rejecting reauth