batches: calculate batch spec diffstats in the database

Created by: LawnGnome

This massively improves performance when previewing batch specs — on a test batch spec locally with +957,887 •298 −0, this takes the time to resolve the diffStat field from 2-4 seconds to tens of milliseconds. (Remember, this query is refreshed every five seconds, by default, when anyone has the preview page open.)

My thanks to @eseliger for adding the right AuthzQueryConds magic to make this work, along with the unit tests.

Test plan

Existing unit tests continue to pass, new tests were added (thanks again, Erik!), and I've tested this locally in browser for functionality and with graphql-field-timer for performance.