Add UI/UX for moving channels (#2976)

TODO:

- [x] Add drag and drop
- [x] Polish up in-flight decisions.
- [x] Fix chat panel panic
- [x] Add nice hover effect highlighting the matching ones
- [x] Fix and test keyboard

Release Notes:

- N/A

crates/collab_ui/src/chat_panel.rs

@@ -166,8 +166,8 @@ impl ChatPanel {
                 let selected_channel_id = this
                     .channel_store
                     .read(cx)
-                    .channel_at_index(selected_ix)
-                    .map(|e| e.1.id);
+                    .channel_at(selected_ix)
+                    .map(|e| e.id);
                 if let Some(selected_channel_id) = selected_channel_id {
                     this.select_channel(selected_channel_id, cx)
                         .detach_and_log_err(cx);
@@ -391,7 +391,7 @@ impl ChatPanel {
             (ItemType::Unselected, true) => &theme.channel_select.hovered_item,
         };
 
-        let channel = &channel_store.read(cx).channel_at_index(ix).unwrap().1;
+        let channel = &channel_store.read(cx).channel_at(ix).unwrap();
         let channel_id = channel.id;
 
         let mut row = Flex::row()