[Admin app] Add ability to add a distribution list as a domain reviewer

Currently only people are supported, but there are cases where distribution list are desirable

The issue identifies the Admin app and its domain-reviewer workflow but names no files or tests. Begin by locating the entry point for adding person reviewers and inspect how reviewer identities are represented. Done means distribution lists can be added as domain reviewers while existing person reviewers continue to work.

Written by the indexing model from the issue text.