This PR fixes an issue where having a lot of participants in a call
could cause the avatars/facepiles to overflow the title bar, pushing the
call controls off-screen.

The participant list will now scroll when it would otherwise exceed the
available space:


https://github.com/zed-industries/zed/assets/1486634/806c77e6-bd4c-4864-8567-92e0960734ee

Release Notes:

- Fixed participant list overflowing the title bar.