Add automated accessibility tests and fix GSA accessibility scan items

[neilmb]

Pull request summary

This adds automated accessibility testing with pa11y to the CircleCI configuration for this project. It also addresses some of the accessibility items that were surfaced on the GSA accessibility scan spreadsheet through some very small template changes.

If successful, this should close #3121 (which is the next-to-last task on #3130) and close #3599.

It is unlikely that the CircleCI scan will pass the first time it runs on this branch, but it should be possible to patch up every one of the pa11y errors that are identified when that runs. Optimally, the pa11y CircleCI test will pass cleanly on this branch.

Reminder - please do the following before assigning reviewer

• [ ] update readme
• [ ] For frontend changes, ensure design review

And make sure that automated checks are ok

• fix houndci feedback
• ensure tests pass
• federalist builds
• no new SNYK vulnerabilities are introduced