R1:30a51dac3686

Clarify registration rules more aggressively when configuring auth

Summary: See private chatter. Make it explicitly clear when adding a provider that anyone who can browse to Phabricator can register.

Test Plan: See screenshots.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7570