Add meta referrer tag to all pages

Summary: Ref T4342. Puts meta="referrer" on everything.

Test Plan: In Safari, used the Charles http proxy to verify this change actually stops referrers from being sent.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4342

Differential Revision: https://secure.phabricator.com/D8712