Add /help slash command (#6650)

* Add /help slash command

* Fix copyright year

* go fmt

* Add help command test at api4

* Fix jasonblais's issue

* Update test code

* Fix unit test issue - keep config unchanged

webapp/components/create_comment.jsx

@@ -203,7 +203,11 @@ export default class CreateComment extends React.Component {
             (data) => {
                 this.setState({submitting: false});
                 if (data.goto_location && data.goto_location.length > 0) {
-                    browserHistory.push(data.goto_location);
+                    if (data.goto_location.startsWith('/') || data.goto_location.includes(window.location.hostname)) {
+                        browserHistory.push(data.goto_location);
+                    } else {
+                        window.open(data.goto_location);
+                    }
                 }
             },
             (err) => {