Don't require one-time tokens to view file resources

Summary:
Ref T10262. This removes one-time tokens and makes file data responses always-cacheable (for 30 days).

The URI will stop working once any attached object changes its view policy, or the file view policy itself changes.

Files with `canCDN` (totally public data like profile images, CSS, JS, etc) use "cache-control: public" so they can be CDN'd.

Files without `canCDN` use "cache-control: private" so they won't be cached by the CDN. They could still be cached by a misbehaving local cache, but if you don't want your users seeing one anothers' secret files you should configure your local network properly.

Our "Cache-Control" headers were also from 1999 or something, update them to be more modern/sane. I can't find any evidence that any browser has done the wrong thing with this simpler ruleset in the last ~10 years.

Test Plan:
  - Configured alternate file domain.
  - Viewed site: stuff worked.
  - Accessed a file on primary domain, got redirected to alternate domain.
  - Verified proper cache headers for `canCDN` (public) and non-`canCDN` (private) files.
  - Uploaded a file to a task, edited task policy, verified it scrambled the old URI.
  - Reloaded task, new URI generated transparently.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10262

Differential Revision: https://secure.phabricator.com/D15642

src/applications/system/controller/PhabricatorRobotsController.php

@@ -32,6 +32,7 @@ final class PhabricatorRobotsController extends PhabricatorController {
 
     return id(new AphrontPlainTextResponse())
       ->setContent($content)
-      ->setCacheDurationInSeconds(phutil_units('2 hours in seconds'));
+      ->setCacheDurationInSeconds(phutil_units('2 hours in seconds'))
+      ->setCanCDN(true);
   }
 }