Refactor the use of `process.env.NODE_ENV` and `…env.CONTEXT`

[tordans]

The code relies on those values in multiple places.

We should have a central place to handle this information. The code should only look at this central place for information.

We should have helper methods that are true/false for …

• development only
• anything that is non-production on fixmyberlin.de (local dev, branch previews, staging preview)
• anything that runs in tests only(?)

  webpack/webpack.common.js:
     20  const BABEL_PLUGINS =
     21:   process.env.NODE_ENV === 'production' ||
     22    process.env.CYPRESS_INTERNAL_ENV === 'production'
  

This can rely on Netlify data like https://docs.netlify.com/site-deploys/overview/#deploy-contexts.