Many improvements to IMAP (and some to SMTP)

- Sending email to gmail doesn't get marked as spam
- A sieve automatically puts emails into folders based on the receiver
  (eg. github@b.enjam.info gets put in the github folder)
- Emails get sent as me@b.enjam.info instead of pipeep@b.enjam.info
- Completely disable postfix's LDA in favor of `dovecot-lda`
- Static password authentication is available so the email password can
  be different from the system password
- TLS is used everywhere (although still using the snakeoil cert)
- DIGEST-MD5 or CRAM-MD5 can be used with a static password file, with
  failbacks to PLAIN with TLS (for PAM auth and some broken clients)