XWiki User ID Should be mapped to Piwik User ID Parameter

Piwik (now Matomo) provides the ability to track user IDs and is designed to facet on visitors by IP or User, so when users are NAT'ed and have the same IP it is useful to have XWiki user's mapped to Piwik.

At the moment this is passed to Piwik as a custom variable. Proposal would involve mapping as described here: https://matomo.org/docs/user-id/