Create a migration to add the missing reference from Article to User

Despite the belongs_to relationship declared in the Article model, until
now there's been no reference from Article to User in the db schema.
Seed a few users in the database and associate each seed article with a user
Modify views to display the email of the last user to update each article
Associate each new or edited article with the current user

After building up the association between articles and users, it is
necessary to link an article to the current user when the article is
created or edited.
Link the Cancel button on the Article new/edit form to the referring page

Formerly, the Cancel button linked to the articles index view. But user could
have landed on the form from one of several referring pages. Now the Cancel
button returns the user to the previous view.