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 chart.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: yelirekim

Maniphest Tasks: T13279

Differential Revision: https://secure.phabricator.com/D20496