Offering to co-maintain Graph Mail

Hi,

I'm offering to co-maintain this module. We're relying on it quite extensively on our projects and have already contributed quite some major fixes.

#3389558: File attachments not working
#3351785: Error: Cannot use object of type Drupal\Core\Render\Markup as array in Drupal\graph_mail\Plugin\Mail\GraphMail::mail()
#3385879: Email not sent when address in format "Name <Address>"
#3383317: Allow emails to be saved in 'Sent items'

If extra hands are needed, more then happy to help!