Build in CI

[Riscky]

This is WIP, but the idea is to build the website in CI, then upload the build to S3 and trigger a deploy. This is one of the components of svsticky/sadserver/issues/327

The current build pipeline does everything except triggering the deploy, which will require some changes in sadserver and aas I also updated npm to the lastest version (nvm install --latest-npm), which fixes a bug in the build, but requires updating Netlify and local environments.

TODO:

• [x] More caching (at least .nvm)
• [x] Use the right triggers (master + PRs)
• [x] Make the workflow trigger on a webhook
• [x] Fix current errors
• [x] Figure out what to do with the master/development paradigm (tags?)
• [x] Trigger an actual deploy: https://github.com/distributhor/workflow-webhook
• [x] Make Netlify happy again (Netlify is using an outdated npm version) (see https://docs.netlify.com/configure-builds/manage-dependencies/#npm)
• [x] Schedule a weekly run to make sure there always is a build in S3
• [x] Trigger from Contentful
• [x] Autocancel previous runs: https://github.com/marketplace/actions/cancel-previous-workflow-runs
• [x] Add instructions to the readme for updating local npm
• [x] Trigger deploy to production
• [x] Autosquash and merge

Things we will do later (and I'll make issues for)

• [x] Post a message to Discord on a failed build: https://docs.github.com/en/actions/learn-github-actions/expressions#failure
• [x] Ping healthchecks.io on scheduled run
• [x] Try to do something about hardcoded npm paths (or pin the npm version)