This commit adds a new `*report-timezone*` dynamic var that should get
bound before dates get parsed (i.e. in a query pipeline
middleware). This allows an easier API `(->Timestamp my-date-string)`
while still ensuring the dates are interpretted correctly.

This is a step toward centralization of our timezone and date
conversion logic. As of this commit there are still separate/similar
logic for handling reporting timezone etc in Pulses, drivers etc.

Fixes #7584, fixes #6402