- Add materialized views for list -> subscriber counts, dashboard chart,
and dashboard aggregate stats that slow down significantly on large
databases (with millions or tens of millions of subscribers). These
slow queries involve full table scan COUNTS().
- Add a toggle to enable caching slow results in Settings -> Performance.
- Add support for setting a cron string that crons and periodically
refreshes aggregated stats in materialized views.
Closes#1019.
