Add a SMTP service for development

[di]

This adds a SMTP service which prints the contents of any email sent to it to stdout, and configures it to be our SMTP server in development.

Previously pyramid_mailer.debug was writing each email to disk on the worker service, which was tedious to use.

I opted to roll our own DebuggingServer because by default, our emails are quoteable-printed encoded, but smtpd.DebuggingServer doesn't decode them before printing, so links and such are hard to use.

[ewdurbin]

seems sound.

at some point down the line we may want to keep recent emails in the DB for administrator viewing as well as deliverability analysis (many MTA services provide hooks now adays).

if we ever do this, the dev service could just be a null op that writes to the DB but does not send.