Issue #3199201: Setting origin_dir configuration to empty populates default
requested to merge issue/stage_file_proxy-3199201:3199201-setting-origindir-configuration into 8.x-1.x
Creates a new setting that allows one to use their public file path as the origin directory for SFP requests. Secretly that's how SFP works now if you set the origin_dir == ''. Instead of performing magic, I'm making it explicit so that people can now use the root of a website as a source to pull from.