Update (optional) Group Configuration urls to use settings.COURSE_KEY_PATTERN

Brings optional Group  Configuration urls in line with others, adheres to changes introduced in https://github.com/edx/edx-platform/commit/d9dd45662ec96d465897f1e0d8e16e681d1db067

review:  @auraz, @polesye 
cc: @andy-armstrong @cahrens