Adds DataModel Mark 22

Fixes #2375

@astralbodies may i bother you with a quick review?. This is [the same PR you've reviewed for 4.4](https://github.com/wordpress-mobile/WordPress-iOS/pull/2376), but resubmitted against develop.

The new data model adds one extra field in the Notification object, so there are no warnings at all.

Thank you sir!