demos: Fix Piwik analytics tracking using the wrong URL
demos: Fix Piwik analytics tracking using the wrong URL

Follow-up to f5f546c1247a61ed8bc4cdf33dcb2cef58aa4847.

The tracking snippet loads <https://piwik.wikimedia.org/matomo.js>,
which fails with HTTP 401 Unauthorized (and pops up an extremely annoying
"Sign in" dialog in browsers when you view the demos). Comparing our
tracking snippet to the one on <https://wikimediafoundation.org/>,
it seems that the correct URL is https://piwik.wikimedia.org/piwik.js>.

I'm also changing a reference to 'matomo.php' on the same domain into
'piwik.php', which also fails similarly after fixing the previous issue.

Piwik was apparently renamed to Matomo recently, but clearly the rename
is incomplete.

Bug: T225882
Change-Id: Id5879d5271f0825e024dd16394d5427d510386a1

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui