People - refine permissions on creating new users

Summary: Fixes T7142. Make old permission mean "make (non-bot) users" and then nuance the UI for those administrators who can make bot accounts.

Test Plan: loaded up admin a with full powers and admin b with restricted powers. noted admin a could make a full user. noted admin b could not make a full user. noted admin b got an error even via clever uri hacking.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7142

Differential Revision: https://secure.phabricator.com/D11702