Synchronous email submission

[Changaco]

Liberapay currently uses Amazon's Simple Email Service to send messages. As far as I can tell SES doesn't support synchronous sending, so the Liberapay web app cannot immediately know if an email address isn't reachable, which is a problem because we want to show an error message to the user in that case.

If Liberapay had its own servers with stable IP addresses (#1727), then we could send the emails ourselves and probably ditch SES entirely after a transition period, thus also solving #274. Ideally we would run an open source Python implementation of JMAP and SMTP, however since JMAP is a relatively new standard there isn't any good Python implementation yet.