R1:ef797494ca89

Add Allowed uris config

Summary:
Kind of a quick look at an idea for T2184

Ref T2184

Test Plan: Make sure the site still loads

Reviewers: epriestley

CC: aran, Korvin, mbishopim3

Maniphest Tasks: T2184

Differential Revision: https://secure.phabricator.com/D6045