R1:439821c7b2cd

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…