Reapply "Constrain redirects to Wikipedia Zero partner-sourced requests."

This reverts commit 28bcaf3b592b8e3fa907dc1f4932df76be8f47f4.

Change-Id: Icf333cbd75b43711b866fa18c0eed2ae57d4dffa
Merge "Reapply "Constrain redirects to Wikipedia Zero partner-sourced requests.""