Rework part generating preview and live API urls

Get rid of cmsUrls configuration in SPA and use simplified configuration for API URLs generation.

• There should be one single host which is used to gather the page model for both the preview and live. For SSR SPA that configuration should be provided by the environment variables and `.env` file (e.g. API_URL = 'http://localhost:8080/site').
• This configuration may still be used by CSR SPAs to determine the preview mode but not host-based.