* MM-69390: expand test coverage for mail package
* MM-69390: migrate from archived gopkg.in/mail.v2 to github.com/wneessen/go-mail
* eliminate encodeRFC2047Word in favour of default SetGenHeader behaviour
* log user_id and channel_id when a plugin cancels an email notification
Matches the modified-by-plugin branch and the analogous push notification hook, which already log both fields on cancellation.
* rename mail to md to avoid package import clash
* gofmt changes
* parse angl-addr and test addr-spec exactly
Jesse Hallam
72250e687e