Fix search.html with newer sphinx versions (#34)

This commit fixes an issue with search.html when building with newer
versions of sphinx. Since the release of sphinx 3.4.0 the search bar
has not worked when using the qiskit_sphinx_theme package. This is
because we were not loading a required script to make the search bar
work with the new sphinx theme. This was done in the upstream
pytorch_sphinx_theme package with:

pytorch/pytorch_sphinx_theme@d033830

which looks like it was itself ported from the sphinx-rtd-theme package
which fixed this same issue with:

readthedocs/sphinx_rtd_theme#1021

This is just porting that change to qiskit_sphinx_theme which at least
fixes the search bar in local builds for me.

qiskit_sphinx_theme/search.html

@@ -10,6 +10,7 @@
 {%- extends "layout.html" %}
 {% set title = _('Search') %}
 {% set script_files = script_files + ['_static/searchtools.js'] %}
+{% set script_files = script_files + ['_static/language_data.js'] %}
 {% block footer %}
   <script type="text/javascript" src="{{ pathto('searchindex.js', 1) }}" defer></script>
   {{ super() }}