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
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