Replace `http_host` parameter with `wwsympa_url_local` parameter (addition to #910)
Created by: ikedas
This PR is based on #910.
Though the http_host
parameter has been obsoleted, many site may set this parameter as the same as host part of wwsympa_url
and they may have to remove it (e.g. see a post on dev list).
Instead of keeping this complicating parameter, we'd be better to introduce the new wwsympa_url_local
parameter to specify proxy upstream URL.