Fix rel external for featured articles on org pages

https://www.pivotaltracker.com/story/show/72420384
Add specialist sector tags to searchable data
Make SearchIndexAddWorkerTest more robust

* Ensure test checks that search_index is called on the instance to index
* Ensure test uses the :add method on Whitehall::SearchIndex instead of
:add_batch directly.
Document different search indexing paths
Fix IE7 alignment/float issue for management list on org pages

I've added in a fixed height of 110px (min-height not IE7 compatible)
to fix alignment issue in IE7. This should be enough for allow longer
names listed and not too much to make the list items too tall and have
too much unecessary white space

https://www.pivotaltracker.com/story/show/70927282
Merge pull request #1576 from alphagov/fix-management-misalignment-IE7

Fix IE7 alignment/float issue for management list on org pages
Ensure specialist sector tags order

* Primary specialist sector tag should be first in index

* Actions PR comments for #specialist_sector_tags test

* Changes Edition#specialist_sector_tags implementation to use
  #primary_specialist_sector_tag and #secondary_specialist_sector_tags
  methods available through SpecialistSector mixin
Move specialist_sector_tags method

* Moves #specialist_sector_tags and #searchable_specialist_sector_tags
  to SpecialistSector module

* Moves related tests to specialist_sectors_test

* Simply #searchable_specialist_sector_tags array creation

* Deletes unused specialist_sectors factory
Merge pull request #1575 from alphagov/add-specialist-sector-tags-to-elasticsearch-export

Add specialist sector tags to elasticsearch export
Merge pull request #1568 from alphagov/fix-featured-rel-external

Fix rel external for featured articles on org pages