Replace SMTP KIO Slave with KSMTP library

Instead of using a KIO Slave for sending emails via SMTP, for which
KIO wasn't really designed, use a KSMTP library. This has no
functional or performance impacts, but improves maintainability
of our SMTP code and opens doors to further improvements (like
native Gmail authentication)

Differential Revision: https://phabricator.kde.org/D6879