Change SpecialistSectors controller to follow REST

* Changes specialist sector route to hit a show action
* Renames SpecialistSectorsController#sector to #show
• Adjusts SpecialistSectorsController before_filter
* Renames sector template to show
Extracts Subcategories controller

* Changes subcategory route to hit a show action
* Moves and renames subcategory template to subcategories/show
Extracts Subcategory model

* Moves logic for building subcategories out into a model

* Adds SpecialistSectorPresenter.build_from_subcategory_content method

* Corrects testing for invalid subcategory tags

* Adds validation to Subcategory model
Extracts SpecialistSector model

* Moves logic out of SpecialistSector controller into model
Remove replaced template file
Changes TAG TYPE constant to remove % literal notation

Percent literal notation can be harder to parse or lead the reader to confuse it with other notations, such as %w
Merge pull request #37 from alphagov/refactor-specialist-sector-controller

Refactor specialist sector controller

https://www.pivotaltracker.com/story/show/75806906