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, saw the value no longer monotonically increasing.

Maniphest Tasks: T13279

Differential Revision: https://secure.phabricator.com/D20879