People - add back "add new user" ui

Summary: This got clobbered in D11547. Revive the code but move it up from the base class to the PeopleList controller which is presumably all the main "admin" views. Fixes T7181.

Test Plan: Saw the button once more on /people/...!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7181

Differential Revision: https://secure.phabricator.com/D11698