Burndown chart for group milestone

• Similar to project milestone burndown charts, have a group milestone burndown chart.
• Should follow the design of https://gitlab.com/gitlab-org/gitlab-ee/issues/3044.
• So should not be implemented before #3044 is created.

added EE Premium UX labels

mentioned in issue #3044

added feature proposal label

@victorwu Is this superseding https://gitlab.com/gitlab-org/gitlab-ee/issues/2999?

mentioned in issue #2999 (closed)

@victorwu Same question as in https://gitlab.com/gitlab-org/gitlab-ee/issues/928#note_36177449, why is this a EE Premium ("Ideal for distributed teams") only feature if also EE Starter ("Ideal for co-located teams") has group level issues and group level labels? For me this is a feature also required by co-located teams.

@victorwu Why do my projects have milestone burndowns and I can create overarching milestones in my group but I can't see a burndown with it? Seems odd.

changed milestone to %10.1

changed milestone to %10.2

@victorwu I agree with @subesokun, this seems more like an EES feature than an EEP only one.

Agree with the above - if group milestones are in EES, they should include the burndown charts as well.

changed milestone to %10.3

removed milestone