automod polish (#456)
Bunch of changes here, getting automod to roughly "v0". Intent is to get
this PR merged, then going forward do proper code review of all future
changes.
New here:
- "distinct value" counters (uses redis HyperLogLog; in-memory is
possibly-huge `map[string]bool`
- persist "flags" in redis
- slack webhook notifications for "new mod actions"
- fixes to exiting rules, and disable some trivial examples from
"default" ruleset
- new trivial/example rules, such as GTUBE spam string, counters
- new rule: interaction churn (follow/unfollow)
- new rule: new account reply promo
- helper command to re-process most-recent N posts from an account
- various helper routines in the rules package