Rewrite using Django

Executive summary: @edds told me to rewrite all the JavaScript in Python so I rewrote all the JavaScript in Python.

This stores repositories and dashboards in a database rather than in a Gist and authenticates with GitHub so that you don't have to mess around with tokens. There is an app running at https://gds-fourth-wall.herokuapp.com/ that you can play with. For that instance you need to be a member of the @alphagov organisation to log in.

Limitations:

- OAuth with GitHub Enterprise doesn't work, because the Heroku instance can't connect to it. I should've seen this coming but didn't :(
- There's no interface to allow people to edit dashboards yet. It'll come at some point, but for now you need to have a Django admin login.