Updates Data Model

1. Note Entity Updates
1.1. Removed ‘account’ relationship
1.2. ‘subject’ attribute is now Transformable
2. Account Entity Updates
2.1. Removed notes relationship
WPAccount: Removes warnings

Verified. Both create methods are always called on the main thread.
WPAccount: Splitting Account Changed Notifications

Now we’ll be sending two different notifications:
- Default WordPressCom Account was added / removed.
WPAccount: Replaces retain in favor of strong keyword