provides for shared link to sitemap.xml.gz 

I have a tool in the robot suite to generate the sitemap from all the slugs in the Solr index. See https://github.com/sul-dlss/gis-robot-suite/blob/master/tools/generate_sitemap.rb. I've uploaded the sitemaps to -prod in the `shared/public` folder.

Also see https://github.com/geoblacklight/geoblacklight/issues/14