OpenPGP encryption for email notifications

When receiving email notifications, if a user has provided a public OpenPGP formatted public key associated with an email account, emails addressed to that account should be encrypted using their public key and formatted accordingly.

For instance, Facebook added OpenPGP support to allow ProtonMail users to receive emails encrypted with their ProtonMail public key (tied to the ProtonMail email account of their choosing): cf. https://protonmail.com/blog/protonmail-facebook-pgp . In this particular example, the PGP format Facebook follows is OpenPGP compatible. For more information by Facebook: https://www.facebook.com/notes/protecting-the-graph/securing-email-communications-from-facebook/1611941762379302 .

Although at this time this would mostly benefit ProtonMail users, I suspect flexibly extending this to other formats might be a boon to other users.

Start by reviewing the issue requirements and the linked ProtonMail and Facebook references for the expected OpenPGP email format. Identify the notification and public-key handling entry points in the repository, then define how encryption applies when a matching key is present and what tests demonstrate the completed behavior.

Written by the indexing model from the issue text.