CallCenter V2

Almost complete replacement of Call Center / Supervisor stats feature. Probably easiest to review the new code by itself instead of as a diff.

Spec: https://docs.google.com/a/dimagi.com/document/d/18f633vrxrTLQOCpL2DaW6BUDyPTqpKRMnyexgw_Nhmw/edit#
This is maybe a good place for a docstring? (Reviewing this file I wasn't sure what this function does/when & how it's supposed to be called.)