Open an existing window if there is one

Summary:
Before this change, opening deep-links or importing files would open new windows.

Instead, just use an existing window if there is one.

Documentation: https://developer.chrome.com/docs/web-platform/launch-handler/

Reviewed By: lawrencelomax

Differential Revision: D48865950

fbshipit-source-id: 80c3fb58f1fcc3b8ace00fd9241bf1e374c8345e

desktop/static/manifest.template.json

@@ -15,6 +15,9 @@
         "sizes": "256x256"
       }
     ],
+    "launch_handler": {
+      "client_mode": "focus-existing"
+    },
     "file_handlers": [
       {
           "action": "/",