[mod] separate index and search routes

This makes it easier to separately handle search and index requests
from a web server or from a reverse proxy.

If a request to index contains a query, a permanent redirect HTTP response
is returned. This should give some level of backwards compatibility
for users that have set a searx instance in their browser's search bar.

searx/templates/__common__/opensearch_response_rss.xml

@@ -4,7 +4,7 @@
      xmlns:atom="http://www.w3.org/2005/Atom">
   <channel>
     <title>Searx search: {{ q|e }}</title>
-    <link>{{ base_url }}?q={{ q|e }}</link>
+    <link>{{ url_for('search', _external=True) }}?q={{ q|e }}</link>
     <description>Search results for "{{ q|e }}" - searx</description>
     <opensearch:totalResults>{{ number_of_results }}</opensearch:totalResults>
     <opensearch:startIndex>1</opensearch:startIndex>