Extend signature expiration time to 7 days

[zachmullen]

This reduces the probability of users timing out on uploads, and from a theoretical security standpoint, isn't materially worse than 1 day.

Note that 7 days is the maximum amount supported by access key signing in S3.

[zachmullen]

Yes, theoretically, a big enough file could still take over 7 days on a slow enough connection. But adding refresh signature logic is a lot more complicated than this change -- not to mention, the inflection point at which it's faster to ship a hard drive than transfer over the network is likely somewhere between 1 and 7 days.