Remove deprecated use of script_files

Fixes sphinx-doc/sphinx#6088

* Add script block
* Fix build error

--- sphinx_rtd_theme/layout.html.orig	2019-02-12 12:37:47.000000000 -0700
+++ sphinx_rtd_theme/layout.html	2019-08-06 09:02:26.103136905 -0600
@@ -29,6 +29,7 @@
   {% endif %}
 
   {# JAVASCRIPTS #}
+  {%- block scripts %}
   <script type="text/javascript" src="{{ pathto('_static/js/modernizr.min.js', 1) }}"></script>
   {%- if not embedded %}
   {# 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 #}
@@ -62,6 +63,7 @@
           href="{{ pathto('_static/opensearch.xml', 1) }}"/>
     {%- endif %}
   {%- endif %}
+  {%- endblock %}
 
   {# CSS #}
   <link rel="stylesheet" href="{{ pathto('_static/' + style, 1) }}" type="text/css" />
--- sphinx_rtd_theme/search.html.orig	2019-02-12 12:37:47.000000000 -0700
+++ sphinx_rtd_theme/search.html	2019-08-06 09:03:25.597117851 -0600
@@ -9,7 +9,10 @@
 #}
 {%- extends "layout.html" %}
 {% set title = _('Search') %}
-{% set script_files = script_files + ['_static/searchtools.js'] %}
+{%- block scripts %}
+    {{ super() }}
+    <script type="text/javascript" src="{{ pathto('_static/searchtools.js', 1) }}"></script>
+{%- endblock %}
 {% block footer %}
   <script type="text/javascript">
     jQuery(function() { Search.loadIndex("{{ pathto('searchindex.js', 1) }}"); });