Make kind, sig labels auto-copy to PRs

[jberkus]

Due to the changes from PR milestone automation (#26), we now have a hard requirement that kind, sig and priority labels need to be populated for PRs. I propose that we add to our automation to make that easy, specifically:

If a PR is linked to an issue, and the PR has no kind, sig, or priority labels, the kind, sig and priority labels from the linked issue should be copied to the PR automatically.

Obstacle: Github's notion of a PR being linked to an issue is rather loose; basically, if the PR is mentioned at all in the issue. This could lead to PRs being tagged that are only mentioned tangentally. Is there a useful way around this?

[enisoc]

I'd be in favor of an explicit command to set the associated issue, like:

/issue #33

[fejta-bot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale.
Stale issues rot after an additional 30d of inactivity and eventually close.

If this issue is safe to close now please do so with /close.

Send feedback to sig-testing, kubernetes/test-infra and/or fejta.
/lifecycle stale

[jberkus]

Closing this because I don't see any reasonable way to accomplish it, unless the SIGs adopt some kind of tagging format.