BLD-1117: Add read-only list of Group Configurations.
Add FEATURE['ENABLE_GROUP_CONFIGURATIONS'] and os.environ['FEATURE_GROUP_CONFIGURATIONS'].
Merge pull request #4267 from edx/tim/feature-group-configurations

BLD-1217: Create feature flag for switching Group Configurations editing on and off
Hide GroupConfiguration behind feature flag in cms/templates/settings.html.
Merge pull request #4316 from edx/anton/group-experiments

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