Add '_form.html.erb' partial for the 'new' and 'show' views

 * The form portion of the 'new' and 'show' views is now housed in
 the partial, _form.html.erb
 * We are using this _form partial since the form code is identical for
 both the 'show' and 'new' view files. This practice agrees with DRY. We
 don't want to repeat the same code in multiple places.
Remove code for the New Article form, source _form partial.

 * The code for the form used to generate a New Article is now stored in
 the _form.html.erb partial.
 * Reference the _form partial using <%= render partial: 'form' %>
Add edit.html.erb file.

 * This file render's the form (using the _form.html.erb partial) that
 allows us to edit our articles.