Add backstop visual regression tests to Travis

[cmc333333]

@xtine made excellent progress on ferreting out visual regressions by using the BackstopJS library. Let's generalize that to prevent issues like #473 (and the countless others of its ilk) from coming up by running visual regressions against our live sites.

Somewhat similar to -parser's integration tests, we'd need to load each site's UI glue code and rebuild the frontend for each. We could run -site with that frontend and tie it to a live API. We could grab a screenshot from the live site as a reference and compare against the newly-generated image.