Restrict access to flexigrid search result view

Prevent the function for generating search results for the flexigrid
table from being accessed except by AJAX. This prevents users from
overriding the expected parameters and generating server errors.

gracedb/search/views.py

@@ -52,6 +52,12 @@ def search(request):
 
             # Get call from template for populating flexigrid table
             if _format == 'F':
+                if not request.is_ajax():
+                    err_msg = ("You have tried to access an internal view "
+                        "which is used for generating JavaScript search "
+                        "results. Set 'results_format' to 'S' in your "
+                        "query parameters.")
+                    return HttpResponseBadRequest(err_msg)
                 # Flex format
                 if query_type == 'S':
                     # Superevent query