[Idea] Automation: Comment with PR checklist when commits are pushed

#### Problem

One of the things identified as an issue in #228 is that it can be hard to verify that all conditions in the PR checklist are met when pull requests go through multiple iterations, both as a pull request author and a reviewer. It's possible that a _new_ checklist could help a pull request author ensure that all conditions are met and give the reviewer confidence.

#### Idea

What if we automate a comment with a blank version of the pull request checklist? The pull request author would then edit the comment to make sure all checklist items were satisfied. The comment would get posted when commits are pushed.

Here's something I found that might be helpful: https://github.com/unsplash/comment-on-pr

If #239 happens, this could get very chatty very fast! Also, if we automate things to the point where everything other than the content changes are pretty well covered, this may be less valuable.

Start by reviewing issues #228 and #239 and the referenced unsplash/comment-on-pr project to understand the checklist and concerns about chatty automation. Done would mean a decided, scoped approach for posting a blank pull request checklist when commits are pushed, including how to avoid excessive comments.

Written by the indexing model from the issue text.