Fix HTML search not working with Sphinx-1.8. (readthedocs#672)

* Fix HTML search not working

Since Sphinx-1.7, Sphinx has exported search options as
`documentation_options.js`.  This starts to refer it instead of
definitions of JavaScript variables.
In addition, this also uses `js_tag()` function which added in 1.8
to support additional attributes for scripts (ex. async attribute).

* Add code comment

sphinx_rtd_theme/layout.html

@@ -191,20 +191,28 @@
 
   {% if not embedded %}
 
-    <script type="text/javascript">
-        var DOCUMENTATION_OPTIONS = {
-            URL_ROOT:'{{ url_root }}',
-            VERSION:'{{ release|e }}',
-            LANGUAGE:'{{ language }}',
-            COLLAPSE_INDEX:false,
-            FILE_SUFFIX:'{{ '' if no_search_suffix else file_suffix }}',
-            HAS_SOURCE:  {{ has_source|lower }},
-            SOURCELINK_SUFFIX: '{{ sourcelink_suffix }}'
-        };
-    </script>
-    {%- for scriptfile in script_files %}
-      <script type="text/javascript" src="{{ pathto(scriptfile, 1) }}"></script>
-    {%- endfor %}
+    {# XXX Sphinx 1.8.0 made this an external js-file, quick fix until we refactor the template to inherert more blocks directly from sphinx #}
+    {% if sphinx_version >= "1.8.0" %}
+      <script type="text/javascript" id="documentation_options" data-url_root="{{ pathto('', 1) }}" src="{{ pathto('_static/documentation_options.js', 1) }}"></script>
+      {%- for scriptfile in script_files %}
+        {{ js_tag(scriptfile) }}
+      {%- endfor %}
+    {% else %}
+      <script type="text/javascript">
+          var DOCUMENTATION_OPTIONS = {
+              URL_ROOT:'{{ url_root }}',
+              VERSION:'{{ release|e }}',
+              LANGUAGE:'{{ language }}',
+              COLLAPSE_INDEX:false,
+              FILE_SUFFIX:'{{ '' if no_search_suffix else file_suffix }}',
+              HAS_SOURCE:  {{ has_source|lower }},
+              SOURCELINK_SUFFIX: '{{ sourcelink_suffix }}'
+          };
+      </script>
+      {%- for scriptfile in script_files %}
+        <script type="text/javascript" src="{{ pathto(scriptfile, 1) }}"></script>
+      {%- endfor %}
+    {% endif %}
 
   {% endif %}