Give "Burndown" charts a more straightforward definition and move all the event stuff into "Activity" charts

Summary:
Depends on D20818. Ref T13279. The behavior of the "burndown" chart has wandered fairly far afield; make it look more like a burndown.

Move the other thing into an "Activity" chart.

Test Plan: {F6865207}

Maniphest Tasks: T13279

Differential Revision: https://secure.phabricator.com/D20819