refactor: use handler_url from xblock runtime, not cms
refactor: use handler_url from xblock runtime, not cms

This is a minor refactoring in order to remove
a code dependency of ./xmodule on ./cms

eduNEXT/edx-platform,edx/edx-platform,edx/edx-platform,eduNEXT/edx-platform,arbrandes/edx-platform,angelapper/edx-platform,arbrandes/edx-platform,edx/edx-platform,angelapper/edx-platform,eduNEXT/edx-platform,eduNEXT/edx-platform,arbrandes/edx-platform,angelapper/edx-platform,angelapper/edx-platform,arbrandes/edx-platform,edx/edx-platform