Add server-level option to filter review request users

[bluekeyes]

Internally, we have several service accounts that have elevated permissions in organizations. When using reviewer assignment for rules that are permission based, I often see service accounts assigned as reviewers, which doesn't make sense. We should add an option (probably at the server level?) to exclude users (by pattern) from review assignment when using the all-users or random-users modes.

[invliD]

Even on a per-policy level it would be convenient to have this as an option. As an example, I have a policy that allows multiple users to approve certain changes. I do want someone to automatically get assigned for reviews (using random-users), but the list of users who can approve should be a superset of the list of users to pick from for the assignment—I want a specific user to never be assigned while retaining approval rights.

I can model this today with an or policy and two copies of the actual policy, one that requests reviews and one that doesn't, but it'd be nice to avoid that duplication.