Added setting "cookie_domain" for third party cookies.
This is useful, when piwik is running on a subdomain, and you want to access the global visitorid (_pk_uid) from your websites.
Maybe it would be also a good idea to adjust the JS API to allow easy access to the global visitorid.
At the moment this is just a proposal, I will contact @mattab .
LGTM, created a new rebased PR to fix the tests: https://github.com/matomo-org/matomo/pull/12632