After chat functionality was removed, this panel became redundant. It
only displayed three notification types: incoming contact requests,
accepted contact requests, and channel invitations.

This PR moves those notifications into the collab experience by adding
toast popups and a badge count to the collab panel. It also removes the
notification-panel-specific settings, documentation, and Vim command.

Before you mark this PR as ready for review, make sure that you have:
- [ ] Added a solid test coverage and/or screenshots from doing manual
testing
- [x] Done a self-review taking into account security and performance
aspects
- [x] Aligned any UI changes with the [UI
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)

Release Notes:

- Removed the notification panel from Zed