www/remark42: stop baking REMARK_URL at build time

Since 1.17.0 the server fills the {% REMARK_URL %} placeholder into
the embedded frontend at serve time, using the REMARK_URL it was
started with. The port was still substituting the placeholder with
http://127.0.0.1:8080 before embedding, which left nothing for the
server to fill in, so the widget pointed at an unreachable host
whenever a page omitted remark_config.host.

• Drop the build-time substitution and the REMARK_URL make variable; remark_url in rc.conf now reaches the frontend as well.
• Document remark_remark_web_root in the rc script.