Adds class to the preview container, so it's easier to target with custom css

Author: skttl

src/Our.Umbraco.DocTypeGridEditor/Web/UI/App_Plugins/DocTypeGridEditor/Views/doctypegrideditor.html

@@ -11,6 +11,7 @@
             </div>
         </div>
         <div ng-if="preview"
+             class="dtge-editor-preview-container"
              style="text-align: left;"
              dtge-bind-html-compile="preview"
              prevent-default>