Rename industry_sectors to specialist_sectors

This makes the necessary changes to Frontend to rename the tag type 
"industry_sector" to "specialist_sector".

Here, I've renamed the existing IndustrySectorsController class to be 
SpecialistSectorsController, and its associated tests, view templates 
and helpers have been moved or renamed too.
Merge pull request #523 from alphagov/rename-industry-sector-to-specialist-sector

Rename industry sector to specialist sector

https://www.pivotaltracker.com/story/show/65625746