Center "Open Chat" button in chat panel (#4157)

The layout of the chat panel wasn't correct but this visual glitch was
being masked by caching, because it seems like Taffy was rendering
things slightly differently when laying out the chat panel as a detached
layout node. This wasn't an issue with caching, but rather an
inconsistency with how Taffy lays things out.

Release Notes:

- N/A

crates/collab_ui/src/chat_panel.rs

@@ -559,6 +559,7 @@ impl Render for ChatPanel {
                 } else {
                     this.child(
                         div()
+                            .full()
                             .p_4()
                             .child(
                                 Label::new("Select a channel to chat in.")