Set <article> on narrative and correct CSS

This also adds `.narrative` alongside the existing (and more brittle)
`div.narrative` selector, to ensure fuller backward compatibility. In
practice, eliminating `div.narrative` would probably be fine. But
preserving it won't hurt anything, either.

Author: karlstolley

_layouts/narrative.html

@@ -2,8 +2,8 @@
 layout: default
 ---
 
-<div class="narrative">
+<article class="narrative">
   <h1 class="text-title">{{ page.title }}</h1>
   <p class="citation"> by {{ page.author }}</p>
   {{ content }}
-</div>
+</article>

_sass/_ed.scss

@@ -664,7 +664,8 @@ p.small {
 }
 
 div.poem,
-div.narrative {
+div.narrative,
+.narrative {
   margin-bottom: 5rem;
 }