Add dmarc reporting

[sophiajt]

DRAFT

The beginnings of reporting support for DMARC. At this stage we're lining up all we need in the report, which will later be formed by aggregating the error events into a report that we can send on.

Some outstanding bits that we'll need to address:

• Where does SpfScope come from?
• Where does the source_ip address come from?
• To report both spf/dkim alignment results, we'll need to remove the current shortcircuiting and let both alignment checks run
• We'll also have a set of policy overrides that can occur. At this time, we don't track these.
• The applied policy will need to get passed in as well