kalastatic a11y: Perform accessibility checks via pa11y

[RobLoach]

This issue is to add kalastatic pa11y to build out pa11y report into the prototype.

a11y is not an afterthought. We should be checking and flagging things early and use automated tools for this purpose.

We recognize these are not a catch all in development and that dedicated, manual testing is required for any project. http://pa11y-kalastatic.at.kalamuna.com/a11y/index.html

https://github.com/kalamuna/kalastatic/issues/324