Remove hash from Message-IDs

Originally, the idea was to get unique IDs per microblog instances so
you could access multiple microblogs using sn2mail. This is pretty
stupid, though. a) Why hash username and password? When your password
changes, you end up with broken threads. b) Why do this at all? If you
want to access multiple microblogs, then you should simply use one
mailbox per microblog.

Closes #2.