Add proper body class for the email editor to mark it a block editor

This class is needed so that some CSS rules (e.g. white background, hidden footer)
are applied for WP admin.
[MAILPOET-5854]

mailpoet/lib/AdminPages/Pages/EmailEditor.php

@@ -69,6 +69,6 @@ class EmailEditor {
     // Enqueue media library scripts
     $this->wp->wpEnqueueMedia();
 
-    echo '<div id="mailpoet-email-editor"></div>';
+    echo '<div id="mailpoet-email-editor" class="block-editor"></div>';
   }
 }