Re-add Piwik Tracking

Simon Spannagelrequested to merge
add_piwik into master

We want to switch to Matomo but have a pending ticket with CERN support on this.

Re-adding the old Piwik for now.

Had to completely override the stu* footer template because there is no clean way to inject own footer code. :/ Docsy 🙄

Once Matomo is up and running, we could go for https://github.com/holehan/hugo-component-matomo

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