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/stores/search_store.jsx

@@ -122,7 +122,7 @@ class SearchStoreClass extends EventEmitter {
 
     updatePost(post) {
         const results = this.getSearchResults();
-        if (results == null) {
+        if (!post || results == null) {
             return;
         }
 

webapp/stores/suggestion_store.jsx

@@ -10,6 +10,7 @@ const ActionTypes = Constants.ActionTypes;
 const COMPLETE_WORD_EVENT = 'complete_word';
 const PRETEXT_CHANGED_EVENT = 'pretext_changed';
 const SUGGESTIONS_CHANGED_EVENT = 'suggestions_changed';
+const POPOVER_MENTION_KEY_CLICK_EVENT = 'popover_mention_key_click';
 
 class SuggestionStore extends EventEmitter {
     constructor() {
@@ -27,6 +28,10 @@ class SuggestionStore extends EventEmitter {
         this.removeCompleteWordListener = this.removeCompleteWordListener.bind(this);
         this.emitCompleteWord = this.emitCompleteWord.bind(this);
 
+        this.addPopoverMentionKeyClickListener = this.addPopoverMentionKeyClickListener.bind(this);
+        this.removePopoverMentionKeyClickListener = this.removePopoverMentionKeyClickListener.bind(this);
+        this.emitPopoverMentionKeyClick = this.emitPopoverMentionKeyClick.bind(this);
+
         this.handleEventPayload = this.handleEventPayload.bind(this);
         this.dispatchToken = AppDispatcher.register(this.handleEventPayload);
 
@@ -71,6 +76,16 @@ class SuggestionStore extends EventEmitter {
         this.emit(COMPLETE_WORD_EVENT + id, term, matchedPretext);
     }
 
+    addPopoverMentionKeyClickListener(id, callback) {
+        this.on(POPOVER_MENTION_KEY_CLICK_EVENT + id, callback);
+    }
+    removePopoverMentionKeyClickListener(id, callback) {
+        this.removeListener(POPOVER_MENTION_KEY_CLICK_EVENT + id, callback);
+    }
+    emitPopoverMentionKeyClick(isRHS, mentionKey) {
+        this.emit(POPOVER_MENTION_KEY_CLICK_EVENT + isRHS, mentionKey);
+    }
+
     registerSuggestionBox(id) {
         this.suggestions.set(id, {
             pretext: '',
@@ -304,6 +319,9 @@ class SuggestionStore extends EventEmitter {
                 this.completeWord(id, other.term, other.matchedPretext);
             }
             break;
+        case ActionTypes.POPOVER_MENTION_KEY_CLICK:
+            this.emitPopoverMentionKeyClick(other.isRHS, other.mentionKey);
+            break;
         }
     }
 }