- Optimizes ViewInBrowser router endpoint

- Optimizes ViewInBrowser class
- Optimizes and updates shortcode link category to use the refactored
  getViewInBrowserUrl() method
- Updates Shortcodes to use the refactored getViewInBrowserUrl() method

lib/API/Endpoints/Newsletters.php

@@ -220,7 +220,7 @@ class Newsletters extends APIEndpoint {
       $newsletter->save();
       $subscriber = Subscriber::getCurrentWPUser();
       $preview_url = NewsletterUrl::getViewInBrowserUrl(
-        NewsletterUrl::ACTION_EDITOR,
+        NewsletterUrl::TYPE_LISTING_EDITOR,
         $newsletter,
         $subscriber
       );
@@ -341,7 +341,7 @@ class Newsletters extends APIEndpoint {
       // get preview url
       $subscriber = Subscriber::getCurrentWPUser();
       $newsletter->preview_url = NewsletterUrl::getViewInBrowserUrl(
-        NewsletterUrl::ACTION_LISTING,
+        NewsletterUrl::TYPE_LISTING_EDITOR,
         $newsletter,
         $subscriber,
         $queue