feat: comment in closed PR when it is blocking an update (github) (#981)

This feature adds a new behaviour to Renovate, where it will now add a comment to any existing closed PRs that are "blocking" currently valid updates. A new API function "ensureComment" has been added, its purpose is so we can ensure there exists only one comment with a certain subject/heading. This lets us prevent duplicates as well as update existing, without having to keep state about it. ensureComment needs porting to gitlab, but should be quite easy.