Enforce 'From' header in all sent emails

The forum is installed at OVH instead of Colt, which means it cannot reach the SMTP server used for basically everything else at Oscaro. Mailjet has been chosen as an SMTP relay. However, they won't relay messages that do not contain a "From" header. Since Sentry is installed on the same server, we need to hack it to add this header in the messages it sends — hence this little PR.