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

src/aphront/default/configuration/AphrontDefaultApplicationConfiguration.php

@@ -56,6 +56,7 @@ class AphrontDefaultApplicationConfiguration
         '(?P<view>info)/(?P<phid>[^/]+)/' => 'PhabricatorFileViewController',
         '(?P<view>view)/(?P<phid>[^/]+)/' => 'PhabricatorFileViewController',
         '(?P<view>download)/(?P<phid>[^/]+)/' => 'PhabricatorFileViewController',
+        'alt/(?<key>[^/]+)/(?<phid>[^/]+)/' => 'PhabricatorFileAltViewController',
         'macro/' => array(
           '$' => 'PhabricatorFileMacroListController',
           'edit/(?:(?P<id>\d+)/)?$' => 'PhabricatorFileMacroEditController',