Improve Differential handling of disabled users

Summary:
We currently allow you to assign code review to disabled users, but
should not.

Test Plan:

• Created revisions with no reviewers and only disabled reviewers, was

appropriately warned.

• Looked at a disabled user handle link, was clearly informed.
• Tried to create a new revision with a disabled reviewer, was rebuffed.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D1429