Clear cache on deploy

[eyeseast]

We're still seeing caching issues, but we're also seeing that clearing the full cache is basically fine. So, let's bust the cache every time we deploy the site to production.

[eyeseast]

Here's a builtin solution: https://www.netlify.com/integrations/cloudflare/cache-purge-build-plugin/

Strictly this plugin triggers the cache purge before deploy (but after build), as this is the only functionality Netlify provides.

I'm not sure if we want to run this post-deploy instead. That might mean we need our own version.

[eyeseast]

Closed via #404