Burndown chart for milestones

Having visibility on project progress is paramount so you know where you stand. At GitLab we are working to introduce a burndown chart in milestones. This first simple version will be focused on open issues count only, but there are already a lot of great ideas to improve it in the future.

We encourage you to look at the issue page for more designs and context on this feature: Burndown chart (gitlab-ee#91)

Give us your feedback so we can improve it together. Thanks!