Add an admin API for reindexing sector editions.

In order to get tag information for published
editions into Rummager (and thence to the services
and information page, among others) when a draft
specialist sector is published, we need to notify
Whitehall so it can resubmit all published editions
tagged with that specialist sector.
Merge pull request #1865 from alphagov/add-topic-reindex-endpoint

Add an admin API for reindexing sector editions.

https://www.pivotaltracker.com/story/show/83216078