BLD-1117, BLD-1217: Add read-only list of Group Configurations, Create feature flag.

Changes in here were previously reviewed in pull requests:
https://github.com/edx/edx-platform/pull/4175/ 
https://github.com/edx/edx-platform/pull/4267

Feature is enabled by setting env variable FEATURE_GROUP_CONFIGURATIONS to non-False.

Please approve merge into master @andy-armstrong @cahrens 
cc: @auraz @polesye @olmar