Complain loudly when someone adds a triage/backport* label to an issue

[gsmet]

It happened a few times lately that one of the triage/backport* labels was added to an issue instead of the corresponding pull request, which led to pull requests being ignored when backporting.

When a backport label is added to an issue (and not a PR), we should:

• add a comment to the issue saying this is not something that should be done
• the comment should explicitly ping the user who added the backport label

[gsmet]

@NicolasGras I thought you might be interested by this one?

[nicolasgras]

@gsmet you can assign it to me, I'm gonna work on it.

[gsmet]

Cool, thanks!