Add SITEURL prepend to header cover images.

Prepend SITEURL to HEADER_COVER and article.header_cover in templates so
that the site can be on a subdirectory, such as http://mywebsite.com/blog.

templates/authors.html

@@ -5,7 +5,7 @@
 {% block header %}
     <!-- Page Header -->
     {% if HEADER_COVER %}
-        <header class="intro-header" style="background-image: url('{{ HEADER_COVER }}')">
+        <header class="intro-header" style="background-image: url('{{ SITEURL }}/{{ HEADER_COVER }}')">
     {% elif HEADER_COLOR %}
         <header class="intro-header" style="background-color: {{ HEADER_COLOR }}">
     {% else %}