Prefer DIGEST-MD5 to PLAIN auth

Prefer DIGEST-MD5 to PLAIN auth. Will try to get to SCRAM-SHA1 soon, but in the mean time DIGEST-MD5 should be prefered (even if it's practically worthless, especially since TLS has been negotiated already).

Also, require encryption for PLAIN auth (doesn't matter b/c Conversations requires TLS, but this way we're somewhat future proof and it's more readable if you're looking through the code and don't realize that TLS is required).