appview/pages/pulls: prevent overscrolling when bottom sheet is open

in mobile view, when the bottom sheet is open, overscrolling the sheet
would cause the underlying page to start scrolling. this adds a bit of
js to the page to avoid this behavior.

Signed-off-by: oppiliappan <[e-mail]>

appview/pages/templates/repo/pulls/pull.html

···
           if (details.open && !isDesktop()) {
             backdrop.classList.remove('opacity-0', 'pointer-events-none');
             backdrop.classList.add('opacity-100', 'pointer-events-auto');
+            document.body.style.overflow = 'hidden';
           } else {
             backdrop.classList.remove('opacity-100', 'pointer-events-auto');
             backdrop.classList.add('opacity-0', 'pointer-events-none');
+            document.body.style.overflow = '';
           }
         }
       };