Info4local publish hook

This is a post-publish hook that's intended to do the following:

For `local_government` relevant documents, send them to a RESTful endpoint in a JSON formatted structure that contains a `tags` array, which is all the Atom feed this document will be visible in based on a combination of:

- document type
- local_government flag
- each topic
- each organisation

So if there are two orgs and 1 topic, there'll be 2 items in the list.

These Atom URLs should be sorted by parameter so they're repeatable and reversible.

The custom REST code will be replaced by alphagov/gds-api-adapters#55 once it's merged, so it's safe to ignore that bit.

This code forms the basis of all future email sending within Whitehall, so the right base is important.

FAO @h-lame @chrismdp 

PS: this branch may move slightly and will definitely be rebased to clean up the dangling WIP commits.