proposal: disallow merge pr labelled evaluating

[aladdin-add]

A PR would likely be merged by accident, when it has not been accepted, while someone have approved.
We can add checking evaluating label to wip plugin(though it's not really wip). or another plugin?

[platinumazure]

Agreed in principle. I think we could either add it to wip (though maybe with a different status check text?), or create a new plugin. Creating a new plugin might not be a bad idea.