This PR adds persistence to the toggle state for the `Show Occupied
Channels` filter.

Also, while driving by, I:
- renamed a few variables to use `occupied` over `active`, which
should've happened in https://github.com/zed-industries/zed/pull/52531.
- extracted `"favorite_channels"` into a global

Self-Review Checklist:

- [X] I've reviewed my own diff for quality, security, and reliability
- [X] Unsafe blocks (if any) have justifying comments
- [X] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [X] Performance impact has been considered and is acceptable

Closes #ISSUE

Release Notes:

- Added persistence to the `Show Occupied Channels` collab toggle.