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/rhs_comment.jsx

@@ -223,6 +223,8 @@ export default class RhsComment extends React.Component {
                 user={this.props.user}
                 status={status}
                 isBusy={this.props.isBusy}
+                isRHS={true}
+                hasMention={true}
             />
         );
 
@@ -291,6 +293,8 @@ export default class RhsComment extends React.Component {
                 height='36'
                 user={this.props.user}
                 isBusy={this.props.isBusy}
+                isRHS={true}
+                hasMention={true}
             />
         );
 
@@ -327,6 +331,8 @@ export default class RhsComment extends React.Component {
                         status={status}
                         user={this.props.user}
                         isBusy={this.props.isBusy}
+                        isRHS={true}
+                        hasMention={true}
                     />
                 );
             }
@@ -447,7 +453,11 @@ export default class RhsComment extends React.Component {
                         <div className='post__body' >
                             <div className={postClass}>
                                 {failedPostOptions}
-                                <PostMessageContainer post={post}/>
+                                <PostMessageContainer
+                                    post={post}
+                                    isRHS={true}
+                                    hasMention={true}
+                                />
                             </div>
                             {fileAttachment}
                             <ReactionListContainer post={post}/>