Add support for accessibility testing

[AlexSkrypnyk]

Use pa11y/pa11y in theme's package.json:

• install package
• provide commands to run the testing
• add as a separate test-a11y step to test type
• add as a separate test-a11y step to ahoy
• add env variable to allow a1y to fail with value set to 0 + update to docs (variable and a11y explanation)
• add report storage in CI to test results

See https://www.drupalgovcon.org/sites/default/files/session/slides/2020-09/2020%20-%20DGC%20Pa11y.pdf