[web] Handle unsaved changes if the modal close button is clicked

Summary: Some feedback from @ashoat in D8139 made me realize that even though clicking 'Back' will prompt an unsaved changes modal if necessary, clicking the default close icon on the modal doesn't. This diff is a really simple change to mirror behavior across both close methods.

Test Plan:
Verified that now both methods of exiting the modal will prompt unsaved changes if there are any.

Reviewers: atul, ashoat

Reviewed By: atul

Subscribers: tomek, ashoat

Differential Revision: https://phab.comm.dev/D8337