PLT-6486 Add an @username button to the profile popover, that puts the username in the post when clicked (#6349)

* PLT-6486 Add an `@username` button to the profile popover, that puts the username in the post when clicked

* PLT-6486 Display `@username` mention on the right text area on center or RHS.

* Disable @mentions from profile popover on searches, mentions and pinned posts. Fix js errors.

* Control undefined post in SearchStore that causes an exception.

webapp/components/at_mention/at_mention.jsx

@@ -11,9 +11,16 @@ import {OverlayTrigger} from 'react-bootstrap';
 export default class AtMention extends React.PureComponent {
     static propTypes = {
         mentionName: PropTypes.string.isRequired,
-        usersByUsername: PropTypes.object.isRequired
+        usersByUsername: PropTypes.object.isRequired,
+        isRHS: PropTypes.bool,
+        hasMention: PropTypes.bool
     };
 
+    static defaultProps = {
+        isRHS: false,
+        hasMention: false
+    }
+
     constructor(props) {
         super(props);
 
@@ -76,6 +83,8 @@ export default class AtMention extends React.PureComponent {
                             user={user}
                             src={Client4.getProfilePictureUrl(user.id, user.last_picture_update)}
                             hide={this.hideProfilePopover}
+                            isRHS={this.props.isRHS}
+                            hasMention={this.props.hasMention}
                         />
                     }
                 >