Let maintainers override "CLA Signed" labels

[ambv]

If there's already a label that says "CLA signed", the bot shouldn't remove it. This is for cases like CCLAs. See history on python/cpython#1734 for an example.

[brettcannon]

The specific case was Łukasz wanted to override the CLA label, and so he added "CLA signed" and then removed "CLA not signed". The order of webhook events can't be known ahead of time, but with enough of a delay then getting the current labels should be enough to check if an overriding label was also added simultaneously.

[hugovk]

Closing because the Knights have been replaced by the CLA bot. See the discussion for more details.

#326