Use constant-time string compare for /info requests

If you GET /info?swiftinfo_sig=<HMAC>&swiftinfo_expires=<TIME>, then
the response contains any admin-only information that's been
registered (via calls to register_swift_info(admin=True)).

The bad news is that the info controller isn't using streq_const_time
to compare the valid HMAC signatures with the passed-in one, leading
to a possible timing attack.

The good news is that this isn't a security hole since there's
absolutely nothing interesting in the admin-only section yet, so even
if an attacker does suss out a valid signature, they don't learn
anything. However, we should still fix this in case anything
interesting ever does get added.

Change-Id: I28b6814def67200ddaa6272e09f6a55fb517fd97
Merge "Use constant-time string compare for /info requests"