Provide a setting which forces all file views to be served from an alternate
domain

Summary:
See D758, D759.

• Provide a strongly recommended setting which permits configuration of an

alternate domain.

• Lock cookies down better: set them on the exact domain, and use SSL-only if

the configuration is HTTPS.

• Prevent Phabriator from setting cookies on other domains.

This assumes D759 will land, it is not effective without that change.

Test Plan:

• Attempted to login from a different domain and was rejected.
• Logged out, logged back in normally.
• Put install in setup mode and verified it revealed a warning.
• Configured an alterate domain.
• Tried to view an image with an old URI, got a 400.
• Went to /files/ and verified links rendered to the alternate domain.
• Viewed an alternate domain file.
• Tried to view an alternate domain file without the secret key, got a 404.

Reviewers: andrewjcg, erling, aran, tuomaspelkonen, jungejason, codeblock
CC: aran
Differential Revision: 760