disallow opening private files (#7165)

- 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.

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