Simplify and improve the "burnup" chart

Summary:

• We incorrectly count resolution changes and other noise as opens / closes.
• Show one graph: open bugs over time (red line minus green line). This and its derivative are the values you actually care about. It is difficult to see the derivative with both lines, but easy with one line.

Test Plan: Looked at burnup chart. Saw charty things. Verified resolution changes no longer make the line move.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T923, T982

Differential Revision: https://secure.phabricator.com/D1945