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)

It feels a bit strange defining the same behaviour (calling the `stub_specialist_sector_topics` method) under two different `Given` blocks. Could we generalise the description so we can use the same step for both the existing scenario and the new one here?