R1:8599145b5eec

Implement more consistent publishing rules for repositories

Summary:
Ref T7298. We are currently inconsistent about when we publish feed, email, notifications, audits and Herald rules.

Specifically, there are two settings which impact these things:

- The "importing" flag, which is set when we're importing old commits.
- The "herald-disabled" flag, which was expanded in scope some time ago and now actually means "disable publishing".

Various parts of the pipeline were checking only one of these…