Allow merging multiple stack notifications into common GitHub PR comment

We sometimes get PRs which affect many different Stacks. Because each Stack posts an individual comment with the proposed run status, the PR conversation section can become bloated and basically unusable. Also, we have hit GitHub API rate limits and the comments may have contributed here.

We would like there to be an option to merge proposed runs into a single PR comment which would gather all proposed runs for given PR. We keep comment content quite short, so the GitHub comment character limit should not be an issue.