Split the left panel into new and old for new room list designs

Though we consider the "room list" to mean the RoomList component specifically, the room list is actually the entire left panel as far as the user is concerned. 

The new proposed designs for the room list modify the whole left panel, so we had might as well break it into new and old now instead of later. This "new" left panel is a bare-bones implementation and meant to only provide the absolute basic feature set to function for those who enable the experimental room list, for whatever reason. This is not intended to be a final implementation, or even remotely close to what it could be. An example of this is the lack of breadcrumbs. Given they are likely to change, they are excluded from this temporary skeleton completely.

This also includes a purple/pink bar between the tag panel and left panel. This is so we can, if needed, differentiate between people who made the mistake of turning on the experimental room list while the overall aesthetic makes it indistinguishable. Once the designs are moderately approved, we can (and definitely should) remove the hideous indicator.

src/components/structures/LoggedInView.tsx

@@ -52,6 +52,7 @@ import {
     hideToast as hideServerLimitToast
 } from "../../toasts/ServerLimitToast";
 import { Action } from "../../dispatcher/actions";
+import LeftPanel2 from "./LeftPanel2";
 
 // We need to fetch each pinned message individually (if we don't already have it)
 // so each pinned message may trigger a request. Limit the number per room for sanity.
@@ -656,6 +657,20 @@ class LoggedInView extends React.PureComponent<IProps, IState> {
             bodyClasses += ' mx_MatrixChat_useCompactLayout';
         }
 
+        let leftPanel = (
+            <LeftPanel
+                resizeNotifier={this.props.resizeNotifier}
+                collapsed={this.props.collapseLhs || false}
+                disabled={this.props.leftDisabled}
+            />
+        );
+        if (SettingsStore.isFeatureEnabled("feature_new_room_list")) {
+            // TODO: Supply props like collapsed and disabled to LeftPanel2
+            leftPanel = (
+                <LeftPanel2 />
+            );
+        }
+
         return (
             <MatrixClientContext.Provider value={this._matrixClient}>
                 <div
@@ -669,11 +684,7 @@ class LoggedInView extends React.PureComponent<IProps, IState> {
                     <ToastContainer />
                     <DragDropContext onDragEnd={this._onDragEnd}>
                         <div ref={this._resizeContainer} className={bodyClasses}>
-                            <LeftPanel
-                                resizeNotifier={this.props.resizeNotifier}
-                                collapsed={this.props.collapseLhs || false}
-                                disabled={this.props.leftDisabled}
-                            />
+                            { leftPanel }
                             <ResizeHandle />
                             { pageElement }
                         </div>