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.

---------

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

crates/collab/src/main.rs

@@ -28,6 +28,9 @@ async fn main() -> Result<()> {
         Some("version") => {
             println!("collab v{VERSION}");
         }
+        Some("migrate") => {
+            run_migrations().await?;
+        }
         Some("serve") => {
             let config = envy::from_env::<Config>().expect("error loading config");
             init_tracing(&config);