[Catalog-sig] hash tags

[Daniel Holth]

Check out https://blake2.net/ ; it is both faster and more secure than
md5. md5 does have to go, no matter how secure it is in this
particular application. SHA2 is the only choice that doesn't require a
long explanation. When this came up a little less than a year ago we
talked about maybe including the SHA2 hash in one of the link
attributes <a href= something="hash"> for the benefit of old clients.