Updated Piwik tracking code

Updated Piwik tracking code

Previous to this commit, the Piwik code on this site was not working.
This updates the Piwik tracking code to code that is working on other
sites, so hopefully this will make Piwik work here once again.

I'm quite sure that I've managed to change every html page in this repo.

Replaced with:

find . -iname "*html" -exec \
sed -i '/<!-- Piwik -->/,/<!-- .*End Piwik.* -->/c\
New code...\
...\
...\' {} \;