- Disallow sharing gitignored files through collab
- Show errors when failing to open files
- Show a warning to followers when view is unshared

/cc @mikaylamaki, let's update this to use your `private_files` config
before merge.


Release Notes:

- Added the ability to prevent sharing private files over collab.

---------

Co-authored-by: Piotr <piotr@zed.dev>
Co-authored-by: Mikayla <mikayla@zed.dev>