(js) Restrict special mailboxes to the top level

Fixes #6963

UI/WebServerResources/js/Mailer/Mailbox.service.js

@@ -571,6 +571,16 @@
       });
   };
 
+  /**
+   * @function $canFolderAs
+   * @memberof Mailbox.prototype
+   * @desc Check if the folder can be set as Drafts/Sent/Trash
+   * @returns true if folder is eligible
+   */
+  Mailbox.prototype.$canFolderAs = function() {
+    return this.type == 'folder' && this.level === 0;
+  };
+
   /**
    * @function $setFolderAs
    * @memberof Mailbox.prototype