(bug 2288) move poll-related RPC endpoints to routing

Also moves two of the existing controllers that have an RPC endpoint to use ->register_rpc ( TextCAPTCHA is a special case )

This includes https://github.com/dreamwidth/dw-free/pull/449, and merging this should involve closing the relevant bug.

I am changing the content type from text/javascript to application/json -- hopefully this will not cause an issue

[ please leave bug open ]