The register is much better to store the HTML themes instead of the
application object.  So this migrates it to the registry object.

Since sphinx_rtd_theme-0.5.0, it supports translations.  But Sphinx core
disallows to enable it because theming framework gives special treatment
for the theme for a long time.

This goes to load it via setuptools at first to enable the translations.

Note: The special treatment for sphinx_rtd_theme (< 0.2.5) is not
removed yet.  But it will be removed in the future release.

.. note::

   $ find sphinx tests LICENSE doc/conf.py -type f -exec sed -i '' -e 's/2007\-20../2007-2021/' {} \;
   $ git co sphinx/locale/**/*.js sphinx/templates/epub3/mimetype