Add an "Accepting reviewers" Herald field for commits

Summary:
See PHI262. Fixes T12578. Although this is a bit niche and probably better accomplished through advisory/soft measures ("Add blocking reviewers") in most cases, it isn't difficult to implement and doesn't create any technical or product tension.

If installs write a rule that blocks commits, that will probably also naturally lead them to an "add reviewers" rule anyway.

Also, allow packages to be hit with the typeahead. They're valid reviewers but previously you couldn't write rules against them, for no actual reason.

Test Plan: Used test console to run this against commits, got sensible results for the field value.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12578

Differential Revision: https://secure.phabricator.com/D18839