Add accessibility scans

[rahearn]

closes #37

• Adds pa11y-ci for running accessibility scans locally and in Github actions
  • locally: ./bin/pa11y-scan
  • Github actions happens on each PR
• Adds shared actions for setting up ruby/node/database and for running a server in the background
• Creates a ci Rails environment, that closely matches production. This helps pa11y and security scans not get tripped up by false positives that happen in development and test environments

Github Action will add a comment to the associated PR if it detects a problem

[github-actions[bot]]

Pa11y Failures detected

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[github-actions[bot]]

Pa11y Failures detected

Show failure message

``` ```

[github-actions[bot]]

Pa11y Failures detected

Show failure message

``` ```

[rahearn]

Should we add the pa11y scan to our default rake task? (I'm inclined to, treat it as part of test suite we expect to pass)

I'm 50/50 on this. I like the default scan to be real quick, and pa11y is maybe a bit slower than I'd like. Maybe a task that does both the default and also runs pa11y that we can get in the habit of using? Or add it to the default and back it out to the full if/when it takes too long? Either way I'll create a new task to do that.