Revert componentDidMount changes on breadcrumbs

Fixes https://github.com/vector-im/riot-web/issues/12990

Regressed in https://github.com/matrix-org/matrix-react-sdk/pull/4313

src/components/structures/LeftPanel.js

@@ -44,7 +44,8 @@ const LeftPanel = createReactClass({
         };
     },
 
-    componentDidMount: function() {
+    // TODO: [REACT-WARNING] Move this to constructor
+    UNSAFE_componentWillMount: function() {
         this.focusedElement = null;
 
         this._breadcrumbsWatcherRef = SettingsStore.watchSetting(