Webgateway json 404 11451

This PR is already opened against develop: https://github.com/openmicroscopy/openmicroscopy/pull/1490 but I'm opening against dev_4_4 at the same time - to review together.
It fixes the webgateway image viewer when deployed with a url prefix, as on gretzky.openmicroscopy.org.uk/omero/webgateway but we also need to check that it doesn't break deployment without the prefix, as on howe.

To test, open the image-viewer, copy and paste the url into a new tab and replace /webclient/ with /webgateway/. Check that the image still loads OK etc.