Introduce channels and move collab popover contents to a collaboration panel (#2828)

### Summary

This PR introduces channels: a new way of starting collaboration
sessions. You can create channels and invite others to join them. You
can then hold a call in a channel, where any member of the channel is
free to join the call without needing to be invited.

Channels are displayed in a new panel called the collaboration panel,
which now also contains the contacts list, and the current call. The
collaboration popover has been removed from the titlebar.

![Screen Shot 2023-08-15 at 9 25 37
AM](https://github.com/zed-industries/zed/assets/326587/0f989dea-7fb7-4d50-9acd-25c8f1c30cd1)


For now, the channels functionality will only be revealed to staff, so
the public-facing change is just the move from the popover to the panel.

### To-do

* User-facing UI
  * [x] signed-out state for collab panel
  * [x] new icon for collab panel
  * [x] for now, channels section only appears for zed staff
* [x] current call section styling
(https://zed-industries.slack.com/archives/C05CJUNF2BU/p1691189389988239?thread_ts=1691189120.403009&cid=C05CJUNF2BU)
* [x] Channel members
* Channels
  * [x] style channel name editor
* [x] decide on a special "empty state" for the panel, when user has no
contacts
* [x] ensure channels are sorted in a consistent way (expose channel id
paths to client)
  * [x] Figure out layered panels UX
  * [x] Change add contacts to be the same kind of tabbed modal
* [x] race condition between channel updates and user fetches
(`ChannelStore::handle_update_contacts`)
* [x] race condition between joining channels and channel update
messages `collab::rpc::channel_updated`)
* [x] don't display mic as muted when microphone share is pending upon
first joining call

Release Notes:

- Moved the collaboration dropdown into its own panel.
- Added settings for disabling the AI assistant panel button.
- Switch to lazily initializing audio output sources
(https://github.com/zed-industries/community/issues/1840,
https://github.com/zed-industries/community/issues/1919)

crates/gpui/src/elements.rs

@@ -48,6 +48,10 @@ pub trait Element<V: View>: 'static {
     type LayoutState;
     type PaintState;
 
+    fn view_name(&self) -> &'static str {
+        V::ui_name()
+    }
+
     fn layout(
         &mut self,
         constraint: SizeConstraint,
@@ -272,8 +276,16 @@ impl<V: View, E: Element<V>> AnyElementState<V> for ElementState<V, E> {
             | ElementState::PostLayout { mut element, .. }
             | ElementState::PostPaint { mut element, .. } => {
                 let (size, layout) = element.layout(constraint, view, cx);
-                debug_assert!(size.x().is_finite());
-                debug_assert!(size.y().is_finite());
+                debug_assert!(
+                    size.x().is_finite(),
+                    "Element for {:?} had infinite x size after layout",
+                    element.view_name()
+                );
+                debug_assert!(
+                    size.y().is_finite(),
+                    "Element for {:?} had infinite y size after layout",
+                    element.view_name()
+                );
 
                 result = size;
                 ElementState::PostLayout {