Add accessibility testing for docs site (http://jrnl.sh)

[wren]

This adds more work towards #1060. It wasn't originally listed in that issue, but #1061 pointed out an accessibility problem (low contrast) on the docs site. This pointed out that we don't currently test the docs site for accessibility, so that's what this PR adds.

It runs pa11y, and outputs the results in junit format (so that circle ci can show us the reports).

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have tested this code locally.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.
• All tests pass.

[wren]

Btw, we should set the accessibility tests to be required until all the issues are fixed. Which, uh, could be a while. 😬

[micahellison]

👓