R1:9d8cdce8e1f2

Make the top-level burndown chart in "Maniphest > Reports" show open tasks, not total tasks

Summary:
Ref T13279. See PHI1491. Currently, the top-level "Burnup Rate" chart in Maniphest shows total created tasks above the X-axis, without adjusting for closures.

This is unintended and not very useful. The filtered-by-project charts show the right value (cumulative open tasks, i.e. open minus close). Change the value to aggregate creation events and status change events.

Test Plan: Viewed top-level chart…