FR: Add option to only make a field editable by an admin

[dploeger]

Currently there is a gap in the functionality:

• User can edit field = false => Nobody can edit the field
• Admin only field = true => Only Admins can view the field

What's missing is:
User can view field, only Admins can edit field

[jdalsem]

Good point, but i never had the usecase (yet) where i needed this combo.