Create new group configurations page

**Ticket**: [BLD-1043](https://openedx.atlassian.net/browse/BLD-1043)
**Previously reviewed PR**: https://github.com/edx/edx-platform/pull/4290
**sandbox:** http://studio.polesye.m.sandbox.edx.org/
The functionality is hidden behind the feature flag introduced in https://github.com/edx/edx-platform/pull/4267

@andy-armstrong , @cahrens - asking for formal permission to land on master.
cc: @auraz @polesye @olmar