Move "Change Username" from weird edit panel to standard object action

Summary: Ref T4065. Make this work in a more standard way which administrators have a reasonable shot at finding and using. See D8662 for discussion.

Test Plan: Changed a user's username.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4065

Differential Revision: https://secure.phabricator.com/D8663