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)

The response to the organisation search always needs to be stubbed, because the call is always made.  I just experimented with parameterising this, and I think it gains us very little (and makes there be lots more code). Also, the first Scenario "viewing documents grouped by content format" doesn't actually check anything to do with grouping documents by content format since groupings were removed a couple of months ago, so needs retitling.

Therefore, I've fixed this by retitling the first scenario to "Scenario: viewing documents in a specialist sub-sector", merging the "Given" and "When" clauses, and renaming the helper method to `stub_specialist_sector_tag_lookups`