Replace Electron with Progressive Web App, deprecate Cockpit support

Fixes #316 (closed), #396 (closed), and #397 (closed). See those issues for context. Renders #223 (closed) moot.

Scope

• Build Progressive Web App (PWA) support into Exosphere
  • Build manifest
    • Do we want to use standalone or minimal-ui? https://developer.mozilla.org/en-US/docs/Web/Manifest/display
    • Does Chrome require a 192x192px image? per https://medium.com/@suncommander/how-to-show-pwa-install-banner-add-to-homescreen-for-your-website-b1fbe6ebfdb5
      • No.
  • Build service worker
• maskable icons
• Implement Phase 1 deprecations below, including:
  • Serve Electron app with deprecation notice that points folks to web app / PWA
  • Stop deploying Cockpit service to new instances, remove associated code from codebase
  • Show deprecation and security notice for Cockpit-based server interactions
• Write instructions for installing PWA in docs/pwa-install.md
• Document how to install/use Cockpit for people who still want it for some reason
• Create issues for implementing phases 2 and 3
  • #462 (closed) and #463

Transition Plan (away from Electron + Cockpit support)

	Phase 0	Phase 1	Phase 2	Phase 3
Timeline	Now	When this MR is merged	30 days after phase 1 begins	1 year after phase 2 begins
PWA	Not offered	Offered	Offered	Offered
Electron App	Fully Supported	Served with deprecation notice that points folks to web app or PWA	Not served, only deprecation notice remains	Not served at all, removed from codebase
Cockpit Service Deployed to New Instances	Yes	No, removed from codebase	No	No
Cockpit Interaction Display	Yes	Served with deprecation and security notice	Not served, removed from codebase	Not served, absent from codebase

Resources and notes

• @bgins' PWA manifest generator: https://flat-magenta-polyester-werewolf.fission.app
  • For now, learning out meta tags recommended by PWA manifest generator because:
    • https://medium.com/@firt/dont-use-ios-web-app-meta-tag-irresponsibly-in-your-progressive-web-apps-85d70f4438cb
    • https://stackoverflow.com/questions/39612354/when-not-to-use-mobile-web-app-capable
    • https://stackoverflow.com/questions/18295927/msapplication-starturl-ignored-in-modern-windows-ui
• Desktop Firefox doesn't support installing a PWA, and people are mad about it
  • https://bugzilla.mozilla.org/show_bug.cgi?id=1407202
  • https://bugzilla.mozilla.org/show_bug.cgi?id=1687806
  • https://en.wikipedia.org/wiki/Progressive_web_application yikes
• https://maskable.app/editor

How to Test

• Confirm that PWA is installable from https://try-dev.exosphere.app on:
  • Linux
    • Chromium
  • Windows
    • Chromium
  • Mac OS
    • Chromium, may need to pass the #enable-desktop-pwas flag (source)
  • Android
    • Firefox
    • Chromium
  • iOS
    • Safari
• After merge to master, confirm that Electron desktop app installed from package shows deprecation warning

Screenshots