Add a basic implementation of a WebAuth to OAuth2 shim. The common iact,
params and fail query flags are respected.

We make use of the ucam_wls library to provide the server implementation
and glue it together in a single Django view.

As well as unit tests, manual integration testing is possible by
visiting http://localhost:8001 in the browser with the development or
production environments running. This provides a simple server using
mod_ucamwebauth to test authentication.

Closes #1