Augment release-monitor for breaking changes

[platinumazure]

I was wondering if it might be worthwhile to augment the release-monitor plugin for breaking changes. Basically, it should create a failing or pending status check for breaking changes unless we know we're about to do a major release.

One possible approach:

1. Create a "major release pending" label that can be added to Release Issues when we are about to do a major release
2. Augment release-monitor to check for PRs with "Breaking:" in the merge commit message and create a failing/pending status check, unless there is a "major release" label on the release issue. Status check message would indicate that the change is breaking and that we cannot merge until we are doing a major release.

[not-an-aardvark]

This sounds reasonable to me, although as far as I know we haven't had a problem with accidentally merging breaking changes in the past. Still, it seems very plausible that someone will make a mistake at some point.