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/utils/channel_intro_messages.jsx

@@ -114,6 +114,7 @@ export function createDMIntroMessage(channel, centeredIntro) {
                         width='50'
                         height='50'
                         user={teammate}
+                        hasMention={true}
                     />
                 </div>
                 <div className='channel-intro-profile'>
@@ -121,6 +122,7 @@ export function createDMIntroMessage(channel, centeredIntro) {
                         <UserProfile
                             user={teammate}
                             disablePopover={false}
+                            hasMention={true}
                         />
                     </strong>
                 </div>