Ref T1049. This is a very basic implementation of sending emails to subscribers of build plans when the build passes or fails. At the moment it doesn't embed any information other than a link to a build (e.g. it doesn't pull in the container, buildable or plan name).

It is probably also missing logic to check whether the subscriber can see the buildable / container that the email is about. Since the only thing that's included right now is a link, it shouldn't be too harmful, but a policy check definitely needs to be added when the logic changes to pull in build information (such as plan and buildable).

Added a mailer implementation that writes the mail information to a Phabricator file and verified that the message arrives correctly (I can't send an actual email out from my development machine).