[Feature request] Encrypt notification emails

Apologies if this has already been requested elsewhere; I couldn't seem to find it.

Request is simple enough: once in a while, one gets emails from notify@keybase.io containing information on invitations and trackers and that sort of stuff. Since keybase already knows the public key for the recipient, why not encrypt those mails?

The issue names no files, tests, or entry points. Start by locating the notification email workflow and how recipient public keys are obtained; done would mean notification emails are encrypted for recipients without breaking invitation and tracker notifications.

Written by the indexing model from the issue text.