PLT-4431 Add post queuing to the webapp (#4800)

* Add post queuing to the webapp

* Add more abstraction

webapp/components/create_post.jsx

@@ -161,10 +161,7 @@ export default class CreatePost extends React.Component {
 
         GlobalActions.emitUserPostedEvent(post);
 
-        PostActions.createPost(post, false,
-            () => {
-                // DO nothing.
-            },
+        PostActions.queuePost(post, false, null,
             (err) => {
                 if (err.id === 'api.post.create_post.root_id.app_error') {
                     // this should never actually happen since you can't reply from this textbox