Add a dmarc email processing endpoint and summary report

[brendanheywood]

• Most of the heavy lifting should be a 3rd party library to parse the dmarc emails. This one looks like a good candidate but worth scanning a bit wider:

https://github.com/liuch/dmarc-srg

• Implement a moodle incoming email handler which wires the moodle emails to the dmarc processor above
• Have a convenient admin settings page which generates the dmarc dns entry and possibly even a check api to make sure it's in place (needs to be somewhat flexible)
• Moodle specific dmarc drill down report
• It should be able to work for any domains including non moodle ones, ie if you have a moodle and a drupal side by side moodle can process them both. Maybe have some config for ips or domains of what is ok