Organisations on subsector pages (not ready for merge)

This implements https://www.pivotaltracker.com/story/show/75807888, which adds a list of organisations which are related to the content in a specialist sub-sector.

Looks like 
![screen shot 2014-08-08 at 10 45 27](https://cloud.githubusercontent.com/assets/73564/3855145/d51d9ed4-1ee0-11e4-80dd-c68fc449f778.png)

I think so, but we want to switch the content_api access to use the same mechanism when we do this.  Therefore, I'd like to merge this PR regardless, and make a new PR (which I've started work on already) to move content_api and rummager access to use that pattern.