[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/__common__/opensearch.xml

@@ -15,4 +15,8 @@
   {% if autocomplete %}
     <Url rel="suggestions" type="application/x-suggestions+json" template="{{ host }}autocompleter?q={searchTerms}"/>
   {% endif %}
+
+  <Url type="application/opensearchdescription+xml"
+      rel="self"
+      template="{{ opensearch_url }}" />
 </OpenSearchDescription>