Track events with New Relic custom metrics

For greater insight into what is happening with event processing, track
events and errors with New Relic custom metric under the
Custom/SendgridEvent path.
Don't require basic auth for the unsubscribe handler
Correct event format

SendGrid has flattened their JSON format for events. Match that change.
Merge branch 'feature/sendgrid_events' into develop-fftf
Merge branch 'develop-fftf' of github.com:fightforthefuture/Platform into develop-fftf

Conflicts:
	app/models/sendgrid_events.rb