Netlify deploy doesn't update site if preview hasn't been regenerated

[audiodude]

Steps to reproduce:

1. Deploy a site to Netlify
2. Edit one of the releases
3. Click deploy again

Expected:

The updated site is deployed

Actual:

The old site preview is deployed.

We have frontend logic for invalidating the preview when the site gets edited, but it only operates on the Edit Site page itself. We should have manage some global state around this.

[audiodude]

I think that the site object returned from the API should have a valid_preview: true/false field. We already call loadSite() when the releases or songs on the page change, and of course it is called when navigating back to the site page from the release edit page. So if we had this value, we could automatically update the Preview and deploy buttons.