CHORE - add Visual Regression Testing

[sturobson]

We need to make sure we start adding visual regression tests so that we confidently update patch, minor, and breaking changes.

As we will also be hosting a CDN version of the styles.css and scripts.js file that will depend on the file path altering when we make a major version change we need to include VRTs as part of the develop and/or build step(s).

There are several resources for this, both paid and completely open-source. I have used [BackstopJS[(https://garris.github.io/BackstopJS/) previously.

Is this something that can be part of the CI build?

[khawkins98]

Just leaving a note, in addition to BackstopJS there's also Percy — I think you've previously mentioned it?

[sturobson]

Closing because of #376