[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/webapp.py

@@ -386,6 +386,9 @@ def render(template_name, override_theme=None, **kwargs):
 
     kwargs['proxify'] = proxify if settings.get('result_proxy', {}).get('url') else None
 
+    kwargs['opensearch_url'] = url_for('opensearch') + '?' \
+        + urlencode({'method': kwargs['method'], 'autocomplete': kwargs['autocomplete']})
+
     kwargs['get_result_template'] = get_result_template
 
     kwargs['theme'] = get_current_theme_name(override=override_theme)