Add pre-creation gl-group-policy-evaluator

Add pre-creation/gl-group-policy-evaluator

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

Split off from https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1702

---

• Going to be used in modules/permissions/lib/ensure-access-and-fetch-descriptor.js(see https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1702) which is used when you create a group
  • https://gitlab.com/gitlab-org/gitter/webapp/blob/b11fb087105fa9044efbc19004758075ad55d463/modules/groups/lib/group-service.js#L153
  • https://gitlab.com/gitlab-org/gitter/webapp/blob/b11fb087105fa9044efbc19004758075ad55d463/modules/groups/lib/group-service.js#L168-169
• Not really related since we are focusing on GL_GROUP here but the code path could be ssed in the room-service to check whether you can create a room for that URI
  • https://gitlab.com/gitlab-org/gitter/webapp/blob/b11fb087105fa9044efbc19004758075ad55d463/modules/rooms/lib/room-service.js#L154-155

TODO

• Wait for https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1709 to merge
• Wait for https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1712 to merge