reverse proxy support

The basic idea is to make sure that leselys will work as a standalone server or behind a reverse proxy, including when configured with a URL prefix like `/leselys/`. This involved wrapping `wsgi_app` in a small custom class to rewrite path info, and modifying quite a bit of javascript and template code to use relative paths instead of rooted paths.

I've done a fair amount of testing, and I think everything still works for standalone servers too. But if you find any problems, just let me know.

Sample nginx config:

        # reverse-proxy for leselys
        rewrite ^/leselys$ /leselys/ permanent;
        location /leselys {
                rewrite ^/leselys(/?.*)$ $1 break;
                proxy_pass     http://127.0.0.1:5000;
                proxy_redirect off;
                proxy_set_header   Host             $host;
                proxy_set_header   X-Real-IP        $remote_addr;
                proxy_set_header   X-Forwarded-For  $proxy_add_x_forwarded_for;
                proxy_set_header   X-Scheme $scheme;
                proxy_set_header X-Script-Name /leselys;
        }