Allow insecure mail auth with "Reply-To" header

Summary: Quora wants to handle some moderation tasks with Phabricator, but want
to lower the barrier to entry for the install and let moderators adopt it
gradually. One request is to allow auth rules to be relaxed so we can auth based
on Reply-To to make things easier. This is insecure if configured but not really
a big deal and the patch isn't big or complicated.

Test Plan: Sent a test email with bogus "From" but valid "Reply-To". It was
rejected with this setting off, and allowed with this setting on.

Reviewers: jungejason, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 842