[DEV-1739] Re-enable 'login/signup to post'

I stashed an id for the newly-created content in the session, read it from the session in a `user_logged_in` signal handler and posted it for real, then stashed the real id in the session, and finally used a piece of URLMiddleware that watches POST requests to the login view to check for the id of the new content and redirect to it if it's there.

But it doesn't require an unsafe/non-idempotent GET request, so it has that going for it.