refactor templates layout

@see https://gohugo.io/templates/new-templatesystem-overview/

layouts/_partials/single/main-header/authors.html

@@ -0,0 +1,25 @@
+{{- /*
+    Renders the authors for an item.
+
+    @params authors  The item's author(s).
+    @params sc  The site context.
+*/ -}}
+
+<span style="display: none">
+<p>Author{{ with .authors }}s{{ end }}:&nbsp;
+{{- with .authors -}}
+    </p>
+    <ul>
+    {{- range . -}}
+        {{- if ( eq . $.site_author.name ) -}}
+            <li><p><a class="p-author" itemprop="author" rel="author" href="{{ $.sc.Site.Home.Permalink }}">{{ $.sc.Site.Params.Author.name }}</a></p></li>
+        {{- else -}}
+            <li><p class="p-author" itemprop="author">{{- . -}}</p></li>
+        {{- end -}}
+    {{- end -}}
+    </ul>
+{{- else -}}
+    <a class="p-author" itemprop="author" rel="author" href="{{ $.sc.Site.Home.Permalink }}">{{ $.sc.Site.Params.Author.name }}</a>
+    </p>
+{{- end -}}
+</span>