Sunset extensions: Open in Editor

Created by: vdavid

Closes https://github.com/sourcegraph/sourcegraph/issues/38378

Added the new settings on the backend, the new UI to the frontend, and a bunch of logic in the background, quite thoroughly unit tested.

Settings are saved on two occasions:

1. When old settings are found and a migration is done
2. When the user presses "Save" on the "Initial setup" popover

Note: Deprecation notices and some thorough testing is missing. I've added these tasks to the testing doc to make sure we don't forget these.

Test plan

• 30+ unit tests
• 5-min Loom where I demo all the functionalities
  • It was originally meant as a UI demo for Sara, but in it, I went through pretty much all use cases, so I'll link it here too.
  • The docs issue demoed in the Loom is already fixed.