Burndown chart for Group milestone view

Description

Our team has switched completely away from Jira. One of the biggest missed features is a "Sprint" (Milestone) burndown chart. We can get burndown charts for each individual project, but there's no main burndown chart that shows all the projects. This is probably been the biggest moving pain from dropping Jira and paying for Gitlab EE.

Proposal

Add a burndown chart to the Milestone view in group issues.

Example of the existing page this burndown chart could be added to: https://gitlab.domain.com/groups/your-group/milestones/sprint-2?title=Sprint+2

Links / references

Documentation blurb

Overview

The burndown chart is need by team leads, project managers, and sprint planners to see the progress of the sprint (milestone). Currently, they can only see the burndown by going to individual projects. Sprint planners need to be able to see the status of all project across the whole sprint, from a bird's eye view.

Use cases

Sprint status checking, by project managers and sprint planners.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.