Move to single table inheritance for company model.
Some updates to ability/authorization file, need to clean up far more.
Remove some unused files.
Large change, Company => RentalCompany
Remove technicians add ContractingCompanies and WorkContract.
Extract user role module from User class.
Add basic user model validations.
Add message/devise and flash/notice styling.
Extract user controller module, reorganize overridden (and duplicate) devise controller and users_controller.