[fix] opensearch.xml URL contains method and autocomplete parameters

When the user add searx as a search engine, the browser loads the /opensearch.xml URL without the cookies.
Without the query parameters, the user preferences are ignored (method and autocomplete).

In addition, opensearch.xml is modified to support automatic updates,
see https://developer.mozilla.org/en-US/docs/Web/OpenSearch

searx/templates/simple/base.html

@@ -29,7 +29,7 @@
           data-no-item-found="{{ _('No item found') }}"></script>
   <!--<![endif]-->
   {% block head %}
-  <link title="{{ instance_name }}" type="application/opensearchdescription+xml" rel="search" href="{{ url_for('opensearch') }}"/>
+  <link title="{{ instance_name }}" type="application/opensearchdescription+xml" rel="search" href="{{ opensearch_url }}"/>
   {% endblock %}
   <link rel="shortcut icon" href="{{ url_for('static', filename='img/favicon.png') }}" />
 </head>