Resolve "Burndown chart for group milestone"

What does this MR do?

Displays Burndown charts for group milestones.

CE version https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18511

TODOs

• Adjust license check to allow only Premium
• Special-case the GroupMilestone link access to not break the page
• Adjust UI alignment for burndown chart
• Show burndown placeholder for group milestones
• Show promotion properly when not available for group milestones

Solves #3064 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #3064 (closed)