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
The next two blocks are quite complex for what seems to be just assigning stub artefacts to Editions. I can't see where having variance in the artefacts would cause the controller or worker to behave differently. Given that generally Publisher no longer makes direct changes to artefacts now (aside from HTTP requests to Panopticon), do we need artefacts initialized here at all?