Add email messaging for the notify command

opentech/apply/activity/messaging.py

...ply/activity/migrations/0052_nullable_by_report_notify.py

opentech/apply/activity/models.py

opentech/apply/activity/options.py

...pply/activity/templates/messages/email/report_notify.html

...h/apply/projects/management/commands/notify_report_due.py

opentech/apply/projects/migrations/0032_report_notified.py

opentech/apply/projects/models.py

opentech/apply/projects/tests/factories.py

opentech/apply/projects/tests/test_commands.py