Implement Phase 2 of Electron + Cockpit Deprecation

Fixes #462 (closed), see that issue for context. Builds on !404 (merged) which should be merged first. Please merge this on 2021-03-31.

How to Test

This is a little tricky. When it's deployed to production, users of distributed Electron app package will see a deprecation notice instead of Exosphere. I promise! 😁

Screenshots

N/A

Post-Merge Checklist

• Remove package downloads from https://try.exosphere.app/packages
• Close Cockpit-related issues on GitLab project, remove cockpit issuelabel