htaccess enhancements

I've added the ``.html`` file extension to the rewrite rules. It’s somewhat silly that ``.htm`` is rewritten, but not ``.html``. Now, we also rewrite .html files if a matching folder exists, like we do with all the other extensions (``.htm``, ``.json``, ``.xml``, ``.atom``, ``.rss``, ``.rdf``, ``.txt``)