Put reverse proxy info in to GeoServer global controls

We have a fix to set geoserver running behind a reverse proxy, but it's just a variable in web.xml

We should have it as an option in GeoServer. See: http://docs.codehaus.org/display/GEOSDOC/How+to+Config

First step is to put as a global option and read and write the appropriate xml.

Next is to put it in to the web admin tool. I'd say put it under Config -> Server.

I know this will be painful, especially knowing wicket, but hopefully the experience will help you think more about how to simplify the design with wicket, and how to make the config system better in the future.