Integrate with Netlify for `/retest` Prow command

[lmktfy]

What would you like to be added:
If there is a Netlify preview for k/website or k/contributor-site, and an org member uses the /retest command, trigger Netlify to rebuild it.

Why is this needed:
There is currently no nice way for a contributor to self-service request a new Netlify test if there has been a flake or similar failure. The author (and only the author) can push an amended commit, but this is a lot of technical work for a simple step.

[lmktfy]

/transfer test-infra

[k8s-ci-robot]

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[lmktfy]

/sig docs

[lmktfy]

/sig testing

[BenTheElder]

Prow has it's own repo now at https://github.com/kubernetes-sigs/prow

Unfortunately github doesn't let us transfer issues across organizations.

[BenTheElder]

Does netlify expose an API for this? If so we should file an issue against kubernetes-sigs/prow and close this one.

If not, I don't think this is feasible.

At a quick glance, I don't think they do exactly. You can trigger deploys but that's not quite it.

[lmktfy]

I think this is in the right place, or at least that Prow is the wrong place. Only part of the change will happen within the Prow code.