Fix sphinx search

Sphinx stopped loading language_data.js globally in 3.4.
This file now has to be loaded explicitly on the search page.

See

- sphinx-doc/sphinx#8445
- readthedocs/sphinx_rtd_theme#1021

doc/_templates/search.html

@@ -3,6 +3,7 @@
 {%- block scripts %}
     {{ super() }}
     <script type="text/javascript" src="{{ pathto('_static/searchtools.js', 1) }}"></script>
+    <script type="text/javascript" src="{{ pathto('_static/language_data.js', 1) }}"></script>
 {%- endblock %}
 {% block body %}
   <h1 id="search-documentation">{{ _('Search') }}</h1>