Consolidate Milestone specific charts | Regressions and Missed Deliverables

The existing regressions and missed-deliverable charts show the number of issues with that label per milestone.

We could merge these charts and display the counts alongside each other using a Stacked, Group, Bar chart:

• https://www.chartjs.org/samples/latest/charts/bar/stacked-group.html

Related comment:

• #51 (comment 114658413)