Fix reverse for webgateway_get_rois_json in /webgateway see #11579

Fixes a rendering error for the webgateway test url /webgateway/ See https://trac.openmicroscopy.org.uk/ome/ticket/11579

To test, go to /webgateway/ E.g. http://gretzky.openmicroscopy.org.uk/omero/webgateway/
Should see empty image viewer template.