Add scripts to the head section

For consumers that also call the add_javascript() function scripts are
added to the _static folder but not loaded in the HTML resulting in them
not working.

This change iterates over the script_files lists and adds the needed
<script></script> element.

Closes-Bug: 1798002
Needed-By: https://review.openstack.org/#/c/607538/
Change-Id: Ie39e138314fed91f59cd9cb5d78d676c04fb78e6

openstackdocstheme/theme/openstackdocs/layout.html

@@ -11,6 +11,9 @@
 {% include 'css.html' %}
 {%- for cssfile in css_files %}
 <link rel="stylesheet" href="{{ pathto(cssfile, 1) }}" type="text/css" />
+{%- endfor %}
+{%- for js in script_files %}
+<script type="text/javascript" src="{{ pathto(js, 1) }}"></script>
 {%- endfor %}
   {# FAVICON #}
 {% if favicon %}