Stagecraft stub

Created a stagecraft stub controller (under: `support/stagecraft_stub`) that will serve JSON for the responses under `support/stagecraft_stub/responses`.

So, a request for `http://dev.gov.uk:3000/stagecraft-stub/licensing/authorities` will serve file `support/stagecraft_stub/responses/licensing/authorities.json`. If a file doesn't exist, it will serve an error JSON.

@sembirayael and @phss