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
Should we be doing any checking here that the edition is still the latest published edition?  This could be being run quite some time after being queued (particularly if panopticon was having problems).