PLT-6890 Fix various scrolling issues (#6727)

* Fix various scrolling issues

* Move reaction scrolling to reaction list

* Handle scrolling when RHS opens

* Only run scroll update code when posts change

webapp/utils/constants.jsx

@@ -1,7 +1,7 @@
 // Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
 // See License.txt for license information.
 
-import keyMirror from 'key-mirror/keyMirror.js';
+import keyMirror from 'key-mirror';
 
 import audioIcon from 'images/icons/audio.png';
 import videoIcon from 'images/icons/video.png';