Composition: Allow refs versions in config

[ndelangen]

Issue: -testing versions is hard- also -users might manually know their list of versions-

What I did

I made it possible for users to specify in main.js their list of versions fr a ref!

// main.js
module.exports = {
  refs: {
    ember: {
      id: 'ember',
      title: 'Ember',
      url: 'https://next--storybookjs.netlify.app/ember-cli',
      versions: {
        next: 'https://next--storybookjs.netlify.app/ember-cli',
        master: 'https://master--storybookjs.netlify.app/ember-cli',
      },
    },
    cra: 'https://next--storybookjs.netlify.app/cra-ts-kitchen-sink',
  },
};

[shilman]

LGTM

[tmeasday]

@ndelangen which takes precedence? The user specified one or the /metadata.json one (assuming both exist)?

I think the user one should (also people might want an easy way to cull the list of versions without manually spec-ing URLs). But this can come later I am thinking.

[ndelangen]

metadata.json would take precedence if it was loaded