Clean up dead code in huxley.core.

This removes the templates, JS, context processors, middleware,
and the single view from the `huxley.core` app, since none of
it is needed for rendering the UI any more.

Test Plan: Load the app, log in, log out. Verify everything looks
okay (other than the things we know are janky).
#248 Error handling.
#248 adjusted error handling.