Gets the HTTP referrer header field.
The referrer is the address of the web page a site visitor was previously
on before arriving at the current page, typically by clicking a link.
Note: When site visitors move from page to page within your site, the referrer property
does not contain the address of the page the site visitor came from. This is because
Wix sites are built as single page applications.
To get the previous page a site visitor was visiting within your site, you can
use wix-storage-frontend to store the visitor's current page
and retrieve the visitor's previous page.