[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/static/plugins/js/infinite_scroll.js

@@ -9,7 +9,7 @@ function loadNextPage() {
         $('#pagination').html('<div class="loading-spinner"></div>');
         $.ajax({
             type: "POST",
-            url: './',
+            url: $('#search_form').prop('action'),
             data: formData,
             dataType: 'html',
             success: function(data) {