(improvement) Add regression testing (for PRs and commits by proxy)

[TheEgghead27]

Frustratingly, we are unable to authorize all Netlify deploy origins for PR previews (and we probably _shouldn't - Google is yelling at me about getting verification for that).
A thought I had was to create deterministic test suites (perhaps with a static "dummy" mirror of API responses that are cached from the real deal) so that we could test updates regardless of login capability (for instance - seeing if the x-date-pickers components still work the same after an update - #46 - a trivial patch and a trivial check, but definitely convenient to automate). With the large amount of development left to do, perhaps these test suites are limited in scope.