* Ensure that views' on_release callbacks are always called (even if
their window is gone), by passing them a `AppContext`, not a
`WindowContext`.
* Fix leaked handles to `CollabPanel`, `NotificationPanel`, and
`ChatPanel` caused by captures in a `ListState` render callback.

This fixes two issues we were seeing with following:
* inability to rejoin a remote project after you closed it
* following not working if a window had previously been closed