Add a CanCDN flag to uploaded files

Summary:
CanCDN flag indicates that a file can be served + cached
via anonymous content distribution networks.

Once D10054 lands, any files that lack the CanCDN flag
will require a one-time-use token and headers will
prohibit cache to protect sensitive files from
unauthorized access.

This diff separates the CanCDN changes from the code that
enforces these restrictions in D10054 so that the changes
can be tested and refined independently.

Test Plan: Work in progress

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: rush898, qgil, epriestley, aklapper, Korvin

Maniphest Tasks: T5685

Differential Revision: https://secure.phabricator.com/D10166