Add capybara-accessible to catch accessibility regressions in CI

[capybara-accessible](https://github.com/casecommons/capybara-accessible) applies javascript accessibility assertions to pages when they load, using Google's [Accessibility Developer Tools](https://github.com/GoogleChrome/accessibility-developer-tools) tests. That way, accessibility errors in markup will trigger errors in CI.

We've fixed one of the layout errors and marked the other failing tests as inaccessible, until they can be addressed.

[Closes #3374]