Refactor /models directory

* Created /concerns directory and moved mixins from /models to /concerns
* Added extend ActiveSupport::Concern to mixins and related code (ex. included do end)
* Created /services directory and added moved files from /models to /services
* Added /mailers directory
* Add folders for STI
* Moved /lib/foreman/model to /models/compute_resources
* Added add directories under /models to config.autoload_paths
* Moved file from /models to /mailers
* Moved file from /models to /helpers

Refactoring only.  NO NEW CODE.  ALL TESTS PASS