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.
If I am not mistaken, both if branches will show a success message. But since we wanted to refactor the redirect messages anyway, you could delete the render_errors_and_action method and change respond_and_redirect_to to accept a flash hash instead of a notice, so the method caller can determine what kind of flash should be displayed.