Exclude specialist sector documents from unified search results

https://www.pivotaltracker.com/story/show/73972778

This changes unified search so that documents with the format "specialist_sector" are excluded from search results. 

To do this, the `UnifiedSearchBuilder` class is changed so that every query includes a `not` filter. Also, lots of unit tests in `unified_searcher_test.rb` and `unified_search_builder_test.rb` have been updated to include the new filter in stub data to keep the tests passing.