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.

Edit: Added remove button for students. Students are allowed to delete theirselves.
Edit2: Removed activate button for students #442 .
Have we already implemented an example like that?