Mark the "Reviewer" field for Commits as deprecated

Summary:
Depends on D18931. Ref T13048. Ref T13041. This field means "the first accepting reviewer, where order is mostly arbitrary". Modern rules should almost certainly use "Accepting Reviewers" instead.

Getting rid of this completely is a pain, but we can at least reduce confusion by marking it as not-the-new-hotness. Add a "Deprecated" group, move it there, and mark it for exile.

Test Plan:
Edited a commit rule, saw it in "Deprecated" group at the bottom of the list:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13048, T13041

Differential Revision: https://secure.phabricator.com/D18932