[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/search.html

@@ -1,4 +1,4 @@
-<form id="search" method="{{ method or 'POST' }}" action="{{ url_for('index') }}">
+<form id="search" method="{{ method or 'POST' }}" action="{{ url_for('search') }}">
   <div id="search_wrapper">
     <div class="search_box">
       <input id="q" autofocus name="q" type="text" placeholder="{{ _('Search for...') }}" tabindex="1" autocomplete="off" spellcheck="false" dir="auto" {% if q %}value="{{ q }}"{% endif %} >