[landing] fix body scrolling when modal is active

Summary:
As I was working on D8477, I noticed that I introduced a regression in D8175 where the page would still scroll even when a modal was active. With @Ashoat's help, we built this diff to fix this regression

{F632177}

Depends on D8477

Test Plan:
Please see the demo video to see that the scroll no longer works when the modal is active, and that the keyserver animations and scroll to top functionality on navigation still works

{F632171}

Reviewers: atul, kamil

Reviewed By: kamil

Subscribers: tomek, ashoat

Differential Revision: https://phab.comm.dev/D8480

landing/landing.react.js

@@ -49,6 +49,19 @@ function LandingSite(): React.Node {
     [modalContext.modals],
   );
 
+  const isModalOpen = modals.length > 0;
+  React.useEffect(() => {
+    const { body } = document;
+    if (!body || !isModalOpen) {
+      return undefined;
+    }
+
+    body.style.overflow = 'hidden';
+    return () => {
+      body.style.overflow = 'auto';
+    };
+  }, [isModalOpen]);
+
   const [showMobileNav, setShowMobileNav] = React.useState<boolean>(false);
 
   const handleResize = React.useCallback(() => {