Make app installable with sw fetch

Make app installable by adding an empty fetch handler to the Service Worker.

Part of https://gitlab.com/gitlab-org/gitter/webapp/-/issues/1992 and gitlab-com/www-gitlab-com#7652 (closed)

We added the web app manifest file in https://gitlab.com/gitlab-org/gitter/webapp/-/merge_requests/1946

Chrome suggesting to install/add to homescreen:	After installing, you can open Gitter in a standalone window and it has a taskbar icon:

Dev notes

• Criteria for being installable: https://web.dev/install-criteria/

• The web app is not already installed
• Meets a user engagement heuristic
• Be served over HTTPS
  • Includes a Web App Manifest that includes:
  • short_name or name
  • icons - must include a 192px and a 512px icon
  • start_url
  • display - must be one of fullscreen, standalone, or minimal-ui
  • Note: prefer_related_applications must not be present, or be false
• Registers a service worker with a functional fetch handler

Testing strategy

1. Enable the service worker by turning on the feature toggle -> http://localhost:5000/api_web/features/web-push/1
2. Visit the site and wait 10 seconds for the Service Worker to be installed, http://localhost:5000
3. Click the add to homescreen button (For Chrome, it's in the browser address bar)
4. 🎉 Standalone Gitter window with taskbar icon