Revert "Improve completion, action and shared project popovers' layout" (#4233)

Reverts zed-industries/zed#4226

Unfortunately there where some worse states possible after this PR.
Reverting until after the launch, as the problems it addresses are still
bad.

![Screenshot 2024-01-23 at 3 20
59 PM](https://github.com/zed-industries/zed/assets/2280405/d25ab0d5-db49-44cb-b865-e320a97616ce)

crates/collab_ui/src/notifications/incoming_call_notification.rs

@@ -125,7 +125,7 @@ impl Render for IncomingCallNotification {
 
         cx.set_rem_size(ui_font_size);
 
-        h_flex().flex_grow().size_full().font(ui_font).child(
+        div().size_full().font(ui_font).child(
             CollabNotification::new(
                 self.state.call.calling_user.avatar_uri.clone(),
                 Button::new("accept", "Accept").on_click({

crates/collab_ui/src/notifications/project_shared_notification.rs

@@ -129,7 +129,7 @@ impl Render for ProjectSharedNotification {
 
         cx.set_rem_size(ui_font_size);
 
-        h_flex().flex_grow().size_full().font(ui_font).child(
+        div().size_full().font(ui_font).child(
             CollabNotification::new(
                 self.owner.avatar_uri.clone(),
                 Button::new("open", "Open").on_click(cx.listener(move |this, _event, cx| {