Add an "Author's projects" Herald field to Differential

Summary:
Ref T1279. This allows installs to implement two different flavors of project review. They can either implement this rule:

When:
  [ ... ] [ ... ]
Take Action:
  [ Add blockign reviewers ] [ Security ]

...which means "every revision matching X needs to be signed off by someone else on the Security team, even if the author is on that team". The alternative is to implement this rule:

When:
  [ Author's projects ] [ do not include ] [ Security ]
  [ ... ] [ ... ]
Take Action:
  [ Add blocking reviewers ] [ Security ]

...which means that people on the Security team don't need a separate signoff from someone else on the team.

I think this weaker version maps to some of what, e.g., Google does (you need to be reviewed by someone with "readability" in a language, but if you have it that's good enough), but I could imagine cases like "Security" wanting to prevent self-review from satisfying the requirement.

@zeeg, not sure which of these use cases is relevant here, but either one should work after this.

Test Plan: Created rules with this field, verified it populated properly in the transcript.

Reviewers: btrahan

Reviewed By: btrahan

CC: zeeg, aran

Maniphest Tasks: T1279

Differential Revision: https://secure.phabricator.com/D7238