Moving custom header above <h1> in article.

This way the "Edit on GitHub" will be above text that is "too
long", i.e. text that runs into it. For example, this happens in
the "How Does Go Compute a Logarithm" post.

templates/_includes/article.html

@@ -1,15 +1,15 @@
 <header>
     {% include '_includes/qr_code.html' %}
+    {% if CUSTOM_HEADER_TEMPLATE %}
+      {{ CUSTOM_HEADER_TEMPLATE.render({"article": article}) }}
+    {% endif %}
     {% if index %}
       <h1 class="entry-title">
         <a href="{{ SITEURL }}/{{ article.url }}">{{ article.title|striptags }}</a>
       </h1>
     {% else %}
       <h1 class="entry-title">{{ article.title|striptags }}</h1>
     {% endif %}
-    {% if CUSTOM_HEADER_TEMPLATE %}
-      {{ CUSTOM_HEADER_TEMPLATE.render({"article": article}) }}
-    {% endif %}
     <p class="meta">
       {% include '_includes/article_time.html' %}
       {% include '_includes/article_stats.html' %}