Make it "easier" to see backstop reference changes

While the GitHub difference tool for images is nice, it does not scale well. It is also hidden on the files tab along with the rest of the code. So we need an easier way for folks to review and approve those changes.

Proposed solution

• We will create a new workflow for PRs separate from the main workflow.
• This workflow will compare the reference images in the PR/commit with the reference images in the PR's target branch.
  • If the PR is to be merged to develop it compares against the develop images
  • If the PR is to be merged to a feature branch it compares against the feature branch images
• The backstop report will always be published to the dev site to design-system-dev.cancer.gov/<PR>/backstop
  • I could be convinced to use a different name from "backstop", I just don't have anything better at this point
• If there are differences, then a a manual workflow step must be approved.
  • When ever a dev pushes new reference images for the PR the images must be re-reviewed.
  • (So don't review it every time if it is a Draft PR.)
  • How we keep track of "did the images change," who knows...
    • Maybe we fingerprint the folder and store that hash in a build file in the uploaded report. Then we can compare that report to the old report?

Tasks

• Complete Storybook testing does not detect outdated reference files #300 along with the above to be submitted in the same PR.