R1:0669abc5f0e1

Use a proper entropy source to generate file keys

Summary:
See T549. Under configurations where files are served from an alternate domain
which does not have cookie credentials, we use random keys to prevent browsing,
similar to how Facebook relies on pseudorandom information in image URIs (we
could some day go farther than this and generate file sessions on the alternate
domain or something, I guess).

Currently, we generate these random keys in a roundabout manner. Instead, use a
real entropy source and…