Uniformly handle byline across all text types

Poem already has `p.byline`, but narrative had `p.citation` (which is
never referenced in the CSS), and drama had no byline at all. The
_Raisin in the Sun_ demo file was inserting the byline directly into the
page content itself. (This commit removes that now-duplicate content.)

So now all templates have a `p.byline`.

Note that poems do not have "by" in front of the author's name, unlike
narratives and drama. The history of that choice in the Ed project is
unclear, but it will continue preserved here.

Author: karlstolley

_layouts/drama.html

@@ -4,5 +4,6 @@
 
 <article class="drama">
   <h1 class="text-title">{{ page.title }}</h1>
+  <p class="byline">by {{ page.author }}</p>
   {{ content }}
 </article>

_layouts/narrative.html

@@ -4,6 +4,6 @@
 
 <article class="narrative">
   <h1 class="text-title">{{ page.title }}</h1>
-  <p class="citation"> by {{ page.author }}</p>
+  <p class="byline">by {{ page.author }}</p>
   {{ content }}
 </article>

_texts/raising.md

@@ -7,8 +7,6 @@ source: "Hansberry, Lorraine, and Robert Nemiroff. A Raisin in the Sun. Rep Rei
 
 ---
 
-<p class="citation"> by {{ page.author }}</p>
-
 (excerpt)
 
 MAMA: Son—how come you talk so much 'bout money?