Placeholder image for Open edX Studio logo, when not running on edx.org

This is for OPEN-306. @explorerleslie said that UX should design a proper logo for Studio that does not use the edX trademark, but for now, I've hacked together a placeholder logo.
Actually, looks like we do import from `lms.envs.common` on line 32, but that's a very small, specific set of variables that we import. None of the other features in this dict are populated from that file.