Add lighthouse bot to evaluate lighthouse score for each PR

@erquhart I think it would be nice to have a lighthouse bot evaluate each PR, giving us SEO, Accessibility score. I've found this as i was researching lighthouse badges, please take a look at [lighthouse-ci](https://github.com/ebidel/lighthouse-ci). I think this must run after the netlify deployment activated on each new PR, because we need a url to run the lighthouse tests against.

What i like about this is it can be configured to display perf, accessibility and seo score, we should keep score of these numbers.
Let me know what do you think about this, and what are the next steps to accomplish this.
I don't have experience setting up these kind of things, but if you don't have time to do it, i would like to give it a try.