automod: action limits; create reports for interaction churn (#465)
The motivation here is to start auto-reporting in production based on
specific rules. Specifically, this PR would start reporting on
interactions churn (follow/unfollow), letting human mods confirm before
taking action on an account.
Before we do that, need to de-duplicate reports. For example, if an
account creates thousands of spammy posts, only want to report the
account once. Generally want to prevent run-away rules from creating
millions of reports, or doing thousands of automated account takedowns
(for example).
This PR prevents re-reporting based on daily counters (fast checks), as
well as double-checking against the mod service API just before filing a
report (slower, but reliable).
This PR also adds "quotas" for mod actions, implemented using the
counter system. This isn't perfect (the counter system itself might be
buggy or broken (causing duplicate actions), but seems like a good
start. If quotas are exceeded, automod will log and skip taking
additional actions until the next day.