Fix Web UI preview in pull requests

Web UI previews are broken due to a missing /elements/ replace in index.html, that is used both locally and for the previews. It seems this problem was introduced by this https://github.com/nuxeo/nuxeo-web-ui/blob/maintenance-3.0.x/index.html#L124.

There a couple of possible solutions to fix the problem: 

• replace the elements on index.html as we do for elements
• prevent the nginx server from replying 404 requests with index.html. by replacing the line with:

try_files $uri $uri/ =404;