Feature/deactivate

This pull request adds a deactivate and a delete button to employer show page.
JobOffers of deactivated employers aren't shown in JobOffer index page.
By deleting an employer, the related job offers, staff members and applications are removed as well.
From what I read here (http://apidock.com/rails/ActiveRecord/Associations/ClassMethods/has_many) a dependent: :destroy should work here and for the job offers, too. Or did you already try it?