Add Percy checks to PR checks

[pixelastic]

Whenever I create a new website with norska, it should add a new check on renovate PRs that compare the new website homepage with the previous one.

This will allow automatic updates that won't break the website.

Because Percy checks are made remotely, I shouldn't need to enable deploy previews for Netlify. It should a be able to send the page to be tested at Percy and run the checks there.

• Think if this is a norska feature or an aberlaas one
• Where to store the Percy account identity?
• The default page to be tested should be the homepage, but could be configured to target a specific test page
• Should only apply to PRs made by renovate

[pixelastic]

I'm dropping this in favor of #131