Add error to RHS reply box for messages > 4000 chars, consistent with create post and edit post errors (#4871)

* functionality

* CSS updates

* cleanup

* moved message length checks to Textbox component

* cleanup

webapp/components/create_post.jsx

@@ -25,7 +25,7 @@ import PreferenceStore from 'stores/preference_store.jsx';
 
 import Constants from 'utils/constants.jsx';
 
-import {FormattedHTMLMessage, FormattedMessage} from 'react-intl';
+import {FormattedHTMLMessage} from 'react-intl';
 import {browserHistory} from 'react-router/es6';
 
 const Preferences = Constants.Preferences;
@@ -61,7 +61,7 @@ export default class CreatePost extends React.Component {
         this.showPostDeletedModal = this.showPostDeletedModal.bind(this);
         this.hidePostDeletedModal = this.hidePostDeletedModal.bind(this);
         this.showShortcuts = this.showShortcuts.bind(this);
-        this.checkMessageLength = this.checkMessageLength.bind(this);
+        this.handlePostError = this.handlePostError.bind(this);
 
         PostStore.clearDraftUploads();
 
@@ -81,6 +81,10 @@ export default class CreatePost extends React.Component {
         };
     }
 
+    handlePostError(postError) {
+        this.setState({postError});
+    }
+
     handleSubmit(e) {
         e.preventDefault();
 
@@ -218,25 +222,6 @@ export default class CreatePost extends React.Component {
         const draft = PostStore.getCurrentDraft();
         draft.message = message;
         PostStore.storeCurrentDraft(draft);
-
-        this.checkMessageLength(message);
-    }
-
-    checkMessageLength(message) {
-        if (message.length > Constants.CHARACTER_LIMIT) {
-            const errorMessage = (
-                <FormattedMessage
-                    id='create_post.error_message'
-                    defaultMessage='Your message is too long. Character count: {length}/{limit}'
-                    values={{
-                        length: message.length,
-                        limit: Constants.CHARACTER_LIMIT
-                    }}
-                />);
-            this.setState({postError: errorMessage});
-        } else {
-            this.setState({postError: null});
-        }
     }
 
     handleUploadClick() {
@@ -335,8 +320,6 @@ export default class CreatePost extends React.Component {
             fullWidthTextBox: PreferenceStore.get(Preferences.CATEGORY_DISPLAY_SETTINGS, Preferences.CHANNEL_DISPLAY_MODE, Preferences.CHANNEL_DISPLAY_MODE_DEFAULT) === Preferences.CHANNEL_DISPLAY_MODE_FULL_SCREEN,
             showTutorialTip: tutorialStep === TutorialSteps.POST_POPOVER
         });
-
-        this.checkMessageLength(this.state.message);
     }
 
     componentDidMount() {
@@ -543,6 +526,7 @@ export default class CreatePost extends React.Component {
                                 onChange={this.handleChange}
                                 onKeyPress={this.postMsgKeyPress}
                                 onKeyDown={this.handleKeyDown}
+                                handlePostError={this.handlePostError}
                                 value={this.state.message}
                                 onBlur={this.handleBlur}
                                 createMessage={Utils.localizeMessage('create_post.write', 'Write a message...')}