This commits adds support for rendering email in text/html,
text/markdown and text/plain inside a MIME/multipart mail.
Bluemonday was added as a dependency and initialized but it is still not
used because the styling of the email is "discarted" too much. But this
needs to be fixed before going to production.
there is no current need to have a separate table for keeping track of
the inboxes, since all emails are getting stored upon arrival and anyone
can access all mails from any rcpt address.
also, added a arrived_at column, as a utc unix timestamp. This serves
the obvious reason to have a ts on an email, but also allows this
service to delete old mail.
