Add view and controller methods to edit an Article

Article#edit manages a view with form elements where and author can alter
an existing article. Validations are performed by Article#update.
Fix article edit form so that it sets the category checkboxes correctly

Formerly, the edit form displayed empty checkboxes for all categories,
regardless of which categories had already been associated with the article.
Now, the checkboxes correctly show these associations. The same form is used
to create new articles, and correctly shows that no categories have been
associated to begin with.
Add links to edit each article from the index views and show article view