Write integration tests for the association between the User and Micropost models, including
the default order in which to display a user's microposts (by their create date descending) and
the automatic removal of a user's microposts when the user is deleted. Add the functionality
necessary to get these specs to pass to both of the aforementioned models - App is now at the point
where it is ready to begin section 10.1.5 (Content validations).