Boost: Detect coming soon or private site and don't nag the user to regenerate critical CSS

[haqadn]

Critical CSS will suggest user to regenerate when the theme has changed, new content is published or plugins are installed. When this happens when the site isn't visible to public, this can confuse users. We should be able to detect and stop prompting the user to regenerate unless the site is public.

This should be easy to do in Atomic. We should also explore and handle it for other common usage.