Add endpoint for registering topic editions.

A companion to https://github.com/alphagov/whitehall/pull/1865, 
in order to get tag information for published
editions into Rummager (and thence to the services
and information page, among others) when a draft
topic is published, Panopticon needs to notify
Publisher so it can resubmit all published editions
tagged with that topic.

All this code can go away when drafts are handled
through the content store pipeline.

https://www.pivotaltracker.com/story/show/83216078
They're as complex as they needed to be to provide a reasonable facsimile of reality, but the tests pass without them so I've removed them.