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 .
I don't think so. But it is not complicated. Just change the function to
 ```
def respond_and_redirect_to(url, flash = nil, action = nil, status = nil)
    respond_to do |format|
      format.html { redirect_to url, flash}
      ...
    end
end
```
and if there are to many calls, that need to be adjusted, you could make it backwards compatible by using this line:
```
 format.html { redirect_to url, flash.is_a?(Hash) ? flash : {success: flash}}
```