Use relative paths for previews to support sites in sub dirs

The use of an absolute path caused the dataobject previewer iframe to
point to an incorrect path when a site was not located directly in the
document root. A 404 page would display in all preview iframes instead
of the preview that had been rendered.

code/DataObjectPreviewer.php

@@ -30,7 +30,7 @@ public function preview(DataObjectPreviewInterface $record)
 
         return sprintf(
             '<div class="dataobjectpreview" data-src="%s"></div>',
-            str_replace(BASE_PATH, '', $htmlFilepath)
+            str_replace(BASE_PATH . '/', '', $htmlFilepath)
         );
     }
 }