[UX] Create more intuitive behavior for "Customize" button to manage displays

Description of the need

From @klonos in https://github.com/backdrop/backdrop-issues/issues/4890

With the label of the action being "Enable" as it currently is, it does indeed not make 100% sense in this case (although there is some logic that supported it being used, and it somewhat kinda matches other similar UI patterns).

Currently, with the PR as it is (and as @olafgrabienski pointed out), there is a small UX WTF when you click "Customize" and the display is moved to the "customized" section (without it actually being customized).

Proposed solution

Here's what @klonos is thinking would be a better UX:

• We leave the button label as "Customize" as per the current PR
• Change the action of that button so that it leads to the "Manage display" page for that display.
• Add a "Cancel" link/button to the "Manage display" form.

Additional information

This is a follow-up to https://github.com/backdrop/backdrop-issues/issues/4890