Refs: #12726

- ikiwiki can't deal with '?' in internal links so we have to use href.
- Using a relative URL in href breaks the offline version.
- So let's use a full blow URL everywhere: https://tails.boum.org/donate/

This should allow us to track it as a custom Piwik campaign parameter.

See:

- https://piwik.org/docs/tracking-campaigns/
- https://piwik.org/faq/how-to/faq_120/