[GSoC21 Application Task] Automatic notification mails using msmtp along with WUI

Implements an automated notification mail system (every notification can be automatically sent as a mail to admin) that can be setup using a WUI located at /plinth/apps/msmtp

MSMTP can also be used by other apps using: plinth.actions.run('msmtp-helper', ['send', '--to', receipent@example.com,'--body',body,'--subject', subject])