View roadmap epics as complete, not started, in progress

• When displaying the epic roadmap, indicate that an epic bar is either complete, not started, or in progress.
• Some epics are not displayed, see https://gitlab.com/gitlab-org/gitlab-ee/issues/3559.
• An epic bar is displayed as:
  • Complete: If all issues in the epic are closed.
  • Not Started: All issues in epic are open.
  • In Progress: At least one issue in the epic is in progress.
• Epic without issues is not displayed, see https://gitlab.com/gitlab-org/gitlab-ee/issues/3559.