Add github projects burndown chart to the PlanetaryDev GitHub project board

[danlatorre]

This seems to be the most popular and actively maintained burndown chart solution for github projects.
https://github.com/thehale/github-projects-burndown-chart

Critical assumptions we'll need to support, as noted on their page:

This tool, while flexible, makes the following assumptions about your project management workflow:

You use one and only one GitHub Project Board for each of your Sprints
You use one and only one GitHub Milestone for each of your User Stories
You use one and only one GitHub Issue for each of your Sprint Backlog Items/Tasks
If you want to track points, each of your GitHub Issues has a label indicating how many points its corresponding task is worth.
Furthermore, all labels that indicate point values have the format .
Multiple labels indicating points on the same Issue are supported.
A Sprint Backlog Task is considered Done if its corresponding GitHub Issue is Closed.