Rename industry sector to specialist sector

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.

*Note: This is branched from the contents of #522, so that pull request should be merged first.*