Switch Phabricator static/user content to specific domain

Summary:
Best practice for user generated content is to use a specific,
identifiable, separate domain. That allows to avoid security
issues like an access to cookies through a shared domain.

Ref T1500.

Test Plan: Ensure Phabricator instances serve files from this new subdomain.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1500

Differential Revision: https://devcentral.nasqueron.org/D2055