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.
Ok, thats strange. But if that is not working it should be possible, to just add a has_many :employers_newsetter_information with a dependent: :destroy