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 .
Small update: it would not work like this. The parameter name should stay notice (as flash is already assigned) and in the format.html line the hash has to be passed to flash:

```
format.html { redirect_to url, flash: (notice.is_a?(Hash) ? notice : {success: notice})}
```