R1:5c1b91ab457d

Consolidate burndown logic into a "BurndownChartEngine"

Summary:
Ref T13279. For now, we need to render burndowns from both Maniphest (legacy) and Projects (new prototype).

Consolidate this logic into a "BurndownChartEngine". I plan to expand this to work a bit like a "SearchEngine", and serve as a UI layer on top of the raw chart features.

The old "ChartEngine" is now "ChartRenderingEngine".

Test Plan:
- Viewed burndowns ("burnups") in Maniphest.
- Viewed burndowns in Projects.
- Saw the same…