Enable more collab UI features (#3496)

* Current Call section of the collab panel
* Improve the collab titlebar
* Add basic UI for following

Following only partially works, but the UI for following is now in
place.

crates/zed2/src/main.rs

@@ -191,7 +191,6 @@ fn main() {
             user_store: user_store.clone(),
             fs,
             build_window_options,
-            call_factory: call::Call::new,
             workspace_store,
             node_runtime,
         });