Simplify Zed's collaboration system by:
- Only allowing member management on root channels.
- Disallowing moving sub-channels between different roots.
- Disallowing public channels nested under private channels.

This should make the mental model easier to understand, and makes it
clearer
who has what access. It is also significantly simpler to implement, and
so
hopefully more performant and less buggy.

Still TODO:
- [x] Update collab_ui to match.
- [x] Fix channel buffer tests.

Release Notes:

- Simplified channel membership management.