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
Move authorization code to shared lib

We will want to authorize endpoints in more than just the datasets app.
We should move the authorization code into a shared location for use
elsewhere.
Add models for Node and NodeType
Add endpoints for organisation manipulation
Add migration for model
Update README.md for multiple apps

We now have multiple apps in Stagecraft and the README needs to be
adapted to convey this.

I've also split up the initialising instructions from the steps to add a
migration for a new model.