Restore/respect "require secure browsing" for Facebook (phabricator)

Summary:
Ref T1536. Because Facebook publishes data from Phabricator to user profiles and that data is sensitive, it wants to require secure browsing to be enabled in order to login.

Respect the existing option, and support it in the UI.

The UI part isn't reachable yet.

Test Plan: {F46723}

Reviewers: chad, btrahan

Reviewed By: chad

CC: arice, wez, aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6224