Burndown chart for group milestone

• Similar to project milestone burndown charts, have a group milestone burndown chart.
• Should account for all issues within the group, including all subgroups nesting down and their projects.
  • That is, any issue with that group milestone should be included in this calculation of the burndown chart, and that means, all those issues could be in many subnested subgroups.
  • This is not yet possible, since https://gitlab.com/gitlab-org/gitlab-ce/issues/36862 is not implemented yet. Will consider that scope in that issue.
  • So the only issues that the group milestone's burndown chart considers is the issues within the projects that are direct children of the given group.

Remove warning message during promotion: