PLT-1598 Slash command works in RHS (#4367)

* PLT-1598 Slash command works in RHS

* fix UserProfile in the reply for Slash Command

* fix some problem about the system messages in RHS
* system message in RHS isn't displayed as comment for root message

* remove status indicator for system message in RHS

* system message in RHS is colored to grey

* system messages don't count as commented post

* fix bug about cleaning draft in RHS

* remove unnecessary function

* implement new model for executing command

webapp/components/post_view/components/post_list.jsx

@@ -293,7 +293,7 @@ export default class PostList extends React.Component {
 
             if (commentRootId) {
                 for (const postId in posts) {
-                    if (posts[postId].root_id === commentRootId) {
+                    if (posts[postId].root_id === commentRootId && !PostUtils.isSystemMessage(posts[postId])) {
                         commentCount += 1;
                         if (posts[postId].user_id === userId) {
                             shouldHighlightThreads = true;