Fix image fullwidth toggle [MAILPOET-1365]

Prevent WordPress $content_width global to override
mailpoet_image_size when calling wp_get_attachment_image_src()
in ACL generated posts.

lib/Config/Initializer.php

@@ -213,7 +213,7 @@ class Initializer {
   }
 
   function setupImages() {
-    add_image_size('mailpoet_newsletter_max', 1320);
+    add_image_size('mailpoet_newsletter_max', Env::NEWSLETTER_CONTENT_WIDTH);
   }
 
   function setupChangelog() {
@@ -301,4 +301,4 @@ class Initializer {
     }
     return WPNotice::displayError($exception);
   }
-}
+}