add new parameter called server.public_instance

for enabling by default advanced limiter functions
in the future allow us to add features just for the public instances

searx/webapp.py

@@ -1294,6 +1294,7 @@ def config():
             },
             'doi_resolvers': list(settings['doi_resolvers'].keys()),
             'default_doi_resolver': settings['default_doi_resolver'],
+            'public_instance': settings['server']['public_instance'],
         }
     )