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#
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