Fix reverse proxy message redirect after web action

[razvanphp]

This small change allows supervisor HTTP server to be served behind a reverse proxy like nginx by fixing the redirect (the only one in web.py).

This is temporary until version 4.0 is launched with the configurable base_path (#593), given the release cycles are so big & there are so much left to do for the python 3 release.

This works very well behind nginx HTTPS offloading without any hack in the configuration:

        location /supervisor/job/ {
                proxy_pass http://10.107.0.10:9001/;
                include includes/proxy-parameters;
        }

I know you already refused this once in PR #495, but I would kindly ask you to reconsider this, it's not breaking anything, there are already only relative links in the whole web interface, so I can't see why this one should not be. This way you actually don't need any base_path anymore.

Related to #29, #827, makes #669 redundant.

[mnaberez]

I know you already refused this once in PR #495, but I would kindly ask you to reconsider this

Please don't open identical duplicate PRs. The place to discuss #495 is on #495.