Reduce what gets linked

[timja]

Fixes #873

Only linkify explicit http and https urls now.

We could relax it a bit if needed, this was the blueocean config:
https://github.com/jenkinsci/blueocean-plugin/blob/2d4a16514c05dace7a5310f399f397c6d8cd2546/jenkins-design-language/src/js/components/Linkify.jsx#L8-L11

Testing done

Pipeline in #873:

Pipeline in #466 (comment):

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