Use current piwik tracking code

* New code is asynchronous
* Remove unneeded extra HTML tags
Add helper convenience function h.site.relative_urls
Better subdomain tracking

If Adhocracy is configured without relative_urls, subdomains are an
essential part of the platform and doesn't represent multiple separate
websites.

From the Piwik docs:

* Track visitors across all subdomains of adhocracy.de

    So if one visitor visits x.adhocracy.de and y.adhocracy.de, they
will
    be counted as a unique visitor.

* Prepend the site domain to the page title when tracking

    So if someone visits the 'About' page on test.adhocracy.de it will
be
    recorded as 'test / About'. This is the easiest way to get an
    overview of your traffic by sub-domain.

* In the "Outlinks" report, hide clicks to known alias URLs

    So clicks on links to Alias URLs (eg. x.domain.com) will not be
    counted as "Outlink".