Consolidate burndown logic into a "BurndownChartEngine"
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".
- Viewed burndowns ("burnups") in Maniphest.
- Viewed burndowns in Projects.
- Saw the same chart.
Reviewed By: amckinley
Maniphest Tasks: T13279
Differential Revision: https://secure.phabricator.com/D20496