CDB-3302 CDB_CheckQuota now also accepts a schema name. Added a convenience method to check if a schema is valid or not

scripts-available/CDB_ExtensionUtils.sql

@@ -11,3 +11,10 @@ BEGIN
   EXECUTE sql;
 END;
 $$ language 'plpgsql' VOLATILE;
+
+CREATE OR REPLACE FUNCTION cartodb.schema_exists(schema_name text)
+RETURNS boolean AS
+$$
+  SELECT EXISTS(SELECT 1 FROM pg_namespace WHERE nspname = schema_name);
+$$
+language sql VOLATILE;