Enable the mail queue to store attachments and send them later

We use a PlaintextService to convert the HTML body to a plaintext
version of it to be able to send it alongside the mail

If a mail is sent via the mail queue, we must also set the sending_time
to prevent the scheduler command controller from sending the same mail
that has been send via the mail queue again.

Add the marker replacement to the subject, so markers can be used
inside the subject, too