Require PR submitters to provide a reason for skipping notes

[jlebon]

Using a skip-notes label is convenient, but humans are creatures of habit and it doesn't take long before reviewers get used to seeing the skip-notes label and scan right past it.

One way to counter this is instead of a label, PR submitters that want to skip release notes have to either provide e.g. a footer in the initial PR description (such as skip-notes: <reason>) or alternatively as a stand-alone comment. This will increase the likelihood that reviewers read the reason and they may then decide to discuss it.

[bgilbert]

We'd need to special-case Dependabot, since it can't add configurable text to PRs. (Or, really, we'd have a configurable list of exempt users.) But that would also be helpful in flagging Dependabot PRs where additional commits have been added without a corresponding release note.