Preflight much earlier (on creating/saving a configuration)

[mmarchetti]

It's possible to do a bunch of work on your project (creating a Python environment, developing your app) only to discover later that something isn't supported (for example, your python version).

We could do a preflight check via a separate API as soon as you create a Deployment, or any time the config file changes.

This Issue is the UX changes to call new API in issue: #487