Make starter content for fresh sites more discoverable

[helen]

Starter content currently only makes an appearance if you open the customizer on a "fresh site" (typically a new install). It's easy for a user to have made other edits before heading into a customizer, which means they'll never see the starter content, and may be left wondering why the theme preview doesn't look anything like their site. On the other hand, if starter content does manage to apply, they might wonder why the customizer preview doesn't look anything like their live site. For the current limited application of starter content, we should consider some kind of prompt to try it out in the customizer before anything else, and show a message in the customizer that starter content has been applied and won't show up on your live site until you hit publish.

More broadly we should make starter content usable in more contexts, opening a ticket for that shortly.