[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/courgette/search.html

@@ -1,7 +1,7 @@
-<form method="{{ method or 'POST' }}" action="{{ url_for('index') }}" id="search_form">
+<form method="{{ method or 'POST' }}" action="{{ url_for('search') }}" id="search_form">
     <div id="search_wrapper">
         <input type="text" autofocus placeholder="{{ _('Search for...') }}" id="q" class="q" name="q" tabindex="1" autocomplete="off" {% if q %}value="{{ q }}"{% endif %}/>
         <input type="submit" value="search" id="search_submit" />
     </div>
     {% include 'courgette/categories.html' %}
-</form>
+</form>