Update the gomail package to overcome connection issues in the mail notifier

[flucrezia]

FEATURE

Email notifier: update mail package from
gopkg.in/gomail.v2
to
gopkg.in/mail.v2 (https://github.com/go-mail/mail)
which is an active fork of the one currently used.

Summary

The current gomail package and smtp package used to try dial connections and to send emails for notification do not work with many, many email provider, included Office365 and others.

Main idea
Simply update the mail package and remove the tryDial() which involves lower level smtp package.

Details

TCP connection in tryDial() hangs for a long long time...With many providers.
Moving to gopkg.in/mail.v2 will solve the issue thanks to an inner timeout.