Let emoji_store.jsx use Client APIs to resolve URLs (#5302)

webapp/stores/emoji_store.jsx

@@ -1,6 +1,7 @@
 // Copyright (c) 2016 Mattermost, Inc. All Rights Reserved.
 // See License.txt for license information.
 
+import Client from '../client/web_client.jsx';
 import AppDispatcher from '../dispatcher/app_dispatcher.jsx';
 import Constants from 'utils/constants.jsx';
 import EventEmitter from 'events';
@@ -148,8 +149,7 @@ class EmojiStore extends EventEmitter {
 
     getEmojiImageUrl(emoji) {
         if (emoji.id) {
-            // must match Client.getCustomEmojiImageUrl
-            return `/api/v3/emoji/${emoji.id}`;
+            return Client.getCustomEmojiImageUrl(emoji.id);
         }
 
         const filename = emoji.filename || emoji.aliases[0];