Use reverse('webgateway') for viewport_server. See #11451

This fixes resolution of various urls in the webgateway image viewer when deployed on a server with a url prefix (as on gretzky: /omero/webgateway/

To test, log in to gretzky as user-1 and go to: 
https://gretzky.openmicroscopy.org.uk/omero/webgateway/img_detail/2958/

Viewer should load and display as normal - Currently no json or image data is loaded (404).