Allow `GitlabClient#milestones` to return group milestones as well
Closes https://gitlab.com/gitlab-org/release-tools/issues/153
https://gitlab.com/gitlab-org/gitlab-ce/issues/41629 was created using this fix.
Closes https://gitlab.com/gitlab-org/release-tools/issues/153
https://gitlab.com/gitlab-org/gitlab-ce/issues/41629 was created using this fix.