Does not work with Matomo

I attach a quickly created patch that makes this module work with the Matomo module that has replaced the now obsolete Piwik module in June 2018.

I have not renamed this module (it is still called Piwik Rules) but I have made minor edits to the admin UI to refer to Matomo. I have not changed the names of internal variables created by this module, as that should be done when renaming it. I have changed references to the error-causing missing piwik_ variables to refer to matomo_. I have also removed a spurious line of code and I have added check_url to sanitise the input.

I have not performed any in-depth security review of this module, but I am happy to use it. Please assist in reviewing or use at your own risk.

I have updated the patch so that it continues to use the original module version number (beta1), rather than incrementing it. I have also added a new admin UI setting that allows this module to override the Matomo URL with a different one, which is particularly useful when handling different hostnames with Matomo possibly installed on multiple servers.

Further update to the patch, making this module more compatible with the Matomo module, correctly setting UserId if that option is turned on in the Matomo module.