Refactor specialist sector controller

In preparation for story https://www.pivotaltracker.com/story/show/75806906

The refactoring has the intention of moving logic out of the controller into adequate models, making it easier to add and test behaviour to the application.
 
Since this was a refactoring, no new tests were added in this step. Pending merge, a separate PR will add test coverage.

Main changes:

* Changes routes be more RESTful
* Adds SubcategoriesController and Subcategory model
* Adds SpecialistSector model
Why not just `"specialist_sector"`?