Return a preview flag in the Page Model API response

This flag may help on the SPA SDK side to differentiate between preview and live modes based on the response. Some of the possible use-cases:

• Debugging your application and configuration. Sometimes we have support tickets saying that manage content buttons are not showing up.
• From the SDK, we don't need to check the existence of the comment nodes before rendering.
• From the SDK, we can pick the right configuration corresponding to the response without having a global state.