For POL lookups do not display the option to "Unlink"

For lookups which are mandatory when an item has been linked there should be no option to "Unlink" the item. Instead, there should be an option to "Replace" the item, as shown in the mockup.
The "Replace" option should open the lookup modal when selected.
The default button style (white background, blue text) should be applied.

This behaviour was generally fixed by ERM-543 but the change doesn't apply for POL lookups until ERM-508 has been done