Add GL_GROUP to security-descriptor-validator

Add GL_GROUP to security-descriptor-validator

Part of https://gitlab.com/gitlab-org/gitter/webapp/issues/2393

Usage is through securityDescriptorService.group.findById in findAssociatedGithubRepoForRoom and findAssociatedGithubObjectForRoom which is used to get the GitHub entity for a given room. We use this for the issue decoration(to grab the correct repo) and so the badger can send the pull request to the correct spot.

Other usage is through and securityDescriptorService.room.findById which is used for all of the policy code(enforces user permissions and actions). It's used to make sure the sd you're basing your policy off of is actually correct and as expected.