[landing] Add link to web app from download page

Summary:
Addresses [ENG-9524](https://linear.app/comm/issue/ENG-9524/add-web-app-to-landing-download-page).

Depends on D13624

Test Plan:
| before | after |
| {F2918873} | {F2918874} |

Reviewers: varun

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13625

landing/download.react.js

@@ -6,6 +6,7 @@ import {
   faApple,
   faWindows,
 } from '@fortawesome/free-brands-svg-icons';
+import { faGlobe } from '@fortawesome/free-solid-svg-icons';
 import { FontAwesomeIcon } from '@fortawesome/react-fontawesome';
 import classNames from 'classnames';
 import * as React from 'react';
@@ -42,6 +43,10 @@ const onClickWindows = () => {
   );
 };
 
+const onClickWeb = () => {
+  window.open('https://web.comm.app', '_blank');
+};
+
 function Download(): React.Node {
   const headerClassName = classNames([typography.heading1, css.heading]);
   const subheaderClassName = classNames([
@@ -88,6 +93,12 @@ function Download(): React.Node {
             <div className={typography.paragraph1}>Windows</div>
           </div>
         </Button>
+        <Button onClick={onClickWeb}>
+          <div className={css.buttonContentContainer}>
+            <FontAwesomeIcon icon={faGlobe} size="lg" />
+            <div className={typography.paragraph1}>Web</div>
+          </div>
+        </Button>
       </div>
     </div>
   );