Encrypt drafts by default

This changes the default behavior of composer. When encryption is
enabled, KMail will store the draft encrypted by default (with option
to turn it off). Previously this feature was off by default.

This is done by renaming NeverEncryptDrafts option to AlwaysEncryptDrafts
to avoid the double negative (!cfg->mNeverEncryptDrafts). For users
who had NeverEncryptDrafts off the behavior will remain the same, as
AlwaysEncryptDrafts is on by default, for all other users this feature
will become enabled and they can choose to turn it off.

Differential Revision: https://phabricator.kde.org/D7189