Support "Review Changes" and "Block Changes" settings for Owners package "Auto Review"

Summary:
Ref T10939. Fixes T8887. This enables and implements the "review" and "blocking review" options for packages.

This is a bit copy-pastey from DifferentialReviewersHeraldAction, which doesn't feel awesome. I think the right fix is Glorious Infrasturcture, though -- I filed T10967 to track that.

Test Plan:

• Set package autoreveiw to "Review".
• Updated, got a reveiwer.
• Set autoreview to "blocking".
• Updated, got a blocking reviewer.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8887, T10939

Differential Revision: https://secure.phabricator.com/D15916