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/create_post.jsx

@@ -774,6 +774,7 @@ export default class CreatePost extends React.Component {
                                 emojiEnabled={window.mm_config.EnableEmojiPicker === 'true'}
                                 createMessage={Utils.localizeMessage('create_post.write', 'Write a message...')}
                                 channelId={this.state.channelId}
+                                popoverMentionKeyClick={true}
                                 id='post_textbox'
                                 ref='textbox'
                             />