[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/simple/infobox.html

@@ -33,7 +33,7 @@
     <div>
       <h3><bdi>{{ topic.name }}</bdi></h3>
       {% for suggestion in topic.suggestions %}
-      <form method="{{ method or 'POST' }}" action="{{ url_for('index') }}">
+      <form method="{{ method or 'POST' }}" action="{{ url_for('search') }}">
         <input type="hidden" name="q" value="{{ suggestion }}">
         <input type="hidden" name="time_range" value="{{ time_range }}">
         <input type="hidden" name="language" value="{{ current_language }}">