Fix Facebook login on mobile violating CSP after form redirect

Summary: Fixes T13254. See that task for details.

Test Plan: Used iOS Simulator to do a login locally, didn't get blocked. Verified CSP includes "m.facebook.com".

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13254

Differential Revision: https://secure.phabricator.com/D20206