Refactor occupations into skill sets

* Renames the occupation model to skill
* Renames the occupation detail model to skill_sets
* Renames the transformers according to new model names
* Removes unused transformers and controllers
* Changes the route names and route urls according to new models
* Creates a route for user/tags that returns the tags for the user skill sets.

Note: During development I kept confusing occupations and occupation details, so I've renamed them to skill and skill sets appropriately.
Signed-off-by: Jesus Fernandez <jesus.fndz@gmail.com>