This makes SubscriberStatisticsRepository the source of truth for
engagement score data instead of having separate queries.
As part of this change, we will now only be displaying the last year's
worth of data when viewing a report for an individual subscriber.
It also updates engagement score calculation to only count human opens,
not machine opens.
MAILPOET-5410
If someone hasn't received at least 3 emails in the last year, we can't
trust that their level of engagement would be the same now as it was
over a year ago.
MAILPOET-5410
