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/actions/global_actions.jsx

@@ -577,3 +577,11 @@ export function postListScrollChange(forceScrollToBottom = false) {
         value: forceScrollToBottom
     });
 }
+
+export function emitPopoverMentionKeyClick(isRHS, mentionKey) {
+    AppDispatcher.handleViewAction({
+        type: ActionTypes.POPOVER_MENTION_KEY_CLICK,
+        isRHS,
+        mentionKey
+    });
+}