Find all articles for category in the controller, rather than in the view

Refactor to move model-access code from the view to the controller.
Clean up category show view

Replace "This is the show view" placeholder text with the category name.
Add an author attribution line and a preview of the content for each article
filed under this category.
Find all categories for the article in the controller rather than in the view

Refactor to move model-access code from the view to the controller.
Clean up the article show view

Replace "This is the show view" placeholder text with the article title.
Add an author attribution line.
Clean up the article index view

Replace "This is the index view" placeholder text and add an author
attribution line for each article.