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