Setting env variables for both prod en preview

[remihuigen]

Is your feature request related to a problem? Please describe.
My env variables for preview and production environments are often out of sync. When adding new vars, I usually forget to do the same for the preview environment

Describe the solution you'd like
Most of my variables are shared between environments. For me personally, it would make sense to set any new variables added to production env, automatically to preview environment. However, this probably isn't the case for everybody.

Alternatively, upon adding (i.e. saving) new vars to production environment, you could show a notification in the UI prompting "copy these variables to preview environment" (or vice versa)

[atinux]

As we are working on the new CF Workers integration, we are keeping this idea in mind :)