archival hashes can be shortened

Original LP Author: Jeff Breidenbach , LP Link: https://bugs.launchpad.net/bugs/987031

The draft for archival hashes for mailman specifies 32 character, base 32 encoding. We can get away with 13 characters which expects the first collision at around 50 billion messages. The the wikipedia charts in their "birthday attack" article. It's not like an occasional collision is a big deal and this is not a security issue.