Site Editor: Connect "Review Changes" modal to Site Editor's back/exit button when there are unsaved changes

[supernovia]

What problem does this address?

When a user has unsaved changes and clicks the site editor back button, they get a generic "Leave Site" warning. It is not obvious to them that they need to Review Changes.

What is your proposed solution?

If they click the back button to exit the site editor while changes are unsaved, show the Review Changes modal.

Current

In a testing session, a user clicked this back button four times and canceled the Leave Site warning four times, unsure of how to escape. We had to point out the "Review Changes" button to them.

Proposed

If they click to exit the editor and there are unsaved changes, instead of showing the browser's default "Leave Site" option, show the Review Changes modal like this: