Allow global Commit herald rules to trigger audits by users

Summary: Ref T4249. Currently, a global rule can only trigger project audits. Although there probably aren't a huge number of use cases for triggering users from global rules, it works fine and it's somewhat confusing not to allow it.

Test Plan: {F90902}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4249

Differential Revision: https://secure.phabricator.com/D7803