Remove add contact buttons once the user logs out
When accessing a page like:
http://rz-vm64.gfz-potsdam.de/devices/204/contacts/new
The user can logout, but will still see the add
button.
We should make this only visible as long as the user is logged in.
For platforms it is the very same.
http://rz-vm64.gfz-potsdam.de/platforms/2/contacts/new
See also #69 (closed).