Added the dtgePreview querystring back in

for the sake of back-compatibility.

Author: leekelleher

src/Our.Umbraco.DocTypeGridEditor/Web/UI/App_Plugins/DocTypeGridEditor/Js/doctypegrideditor.resources.js

@@ -35,7 +35,7 @@
                 );
             },
             getEditorMarkupForDocTypePartial: function (pageId, id, editorAlias, contentTypeAlias, value, viewPath, previewViewPath, published) {
-                var url = umbRequestHelper.convertVirtualToAbsolutePath("~/umbraco/backoffice/DocTypeGridEditorApi/DocTypeGridEditorApi/GetPreviewMarkup?pageId=" + pageId);
+                var url = umbRequestHelper.convertVirtualToAbsolutePath("~/umbraco/backoffice/DocTypeGridEditorApi/DocTypeGridEditorApi/GetPreviewMarkup?dtgePreview=1&pageId=" + pageId);
                 return $http({
                     method: 'POST',
                     url: url,