Close #873, add log codes for better tests, docs, and communication

[plocket]

In this PR, I have:

• [x] Added tests for any new features or bug fixes (if relevant)
• [x] Added my changes to the CHANGELOG.md at the top, under the "Unreleased" section
• [x] Ensured issues that this PR closes will be automatically closed

Reason for this PR

Add log codes. Also:

• Add automatic log code validation and information script.
• Update action and workflow dependencies.
• Fix inaccurate log on failure for docker container setup.
• Neutralize internal a11y tests, which are failing again because of changes to docassemble (that ironically actually are increasing accessibility!). We will trigger those tests manually from now on. This does make testing manually tricky now because I think we have to use a hack like not @none if we want to test all of the tests at once, but I didn't have time to troubleshoot that one.

Links to any solved or related issues

Closes #873

Any manual testing I have done to ensure my PR is working

Ran the new code-validating script locally to make sure it catches different cases.

Additional notes:

GitHub actions are a pain. https://rhysd.github.io/actionlint/ is helpful, as is tmate, but oh boy.