Add accessibility checks

[mgifford]

Issue Report

GitHub gov repositories should also be checked with automated tools for accessibility. We've outlined the process here https://accessibility.civicactions.com/posts/automated-accessibility-testing-leveraging-github-actions-and-pa11y-ci-with-axe

You can see it in action and also see how we're checking for broken links by checking out the code https://github.com/CivicActions/accessibility

Expected behavior

Testing with Pa11y+axe should be integrated in to stop errors from being introduced in a commit.

Actual behavior

I don't think there are any existing tests for this.