Update articlescontainer.class.php to allow using numbers at start of tag to be treated still as tag

Changed to use filter_var when available so tags/strings that start with a number are still evaluated false as an int vs showing as a full int and being treated as a user id.

Currently if a tag starts with a number it is treated as a user id due to have intval casts. As filter_var was added later than MODX min requirements, added conditional logic to maintain backwards compatibility but allow proper validation in future versions.