Summary:
Parent dropdown under Naviagation tab still visible to user groups where it shouldn't be
Detailed Description:
When editing a page, under the Navigation tab, the Parent dropdown still shows
to content editors who shouldn't have permission to see it. It doesn't actually
do anything, but it is there.
Additionally, I would suggest that when a user has no editable fields under a
tab (e.g. they're all hidden with admin_groups=, the tab itself should be hidden
from them.