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)

Would it be worthwhile building this in a similar pattern [to this](https://github.com/alphagov/collections-publisher/blob/8b3a6fb208a818a4b02aacd946775837d7456a1e/config/initializers/services.rb)? /cc @elliotcm