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/post_view/post_header/post_header.jsx

@@ -91,6 +91,7 @@ export default class PostHeader extends React.PureComponent {
                 displayNameType={this.props.displayNameType}
                 status={this.props.status}
                 isBusy={this.props.isBusy}
+                hasMention={true}
             />
         );
         let botIndicator;