Closes#11195. We add a management command to allow us to send emails
to the email mirror directly. The command doesn't require any
configuring of email sending or receiving for the email mirror,
it passes the emails directly using the process_message function.
Fixes part 1 of #10612. We use a regex to remove RE:, FWD: (and similar
variations) from email subjects. Unit test is included, we add
subjects.json in fixtures containing various subjects to try the
stripping on.
