Gets the HTTP referrer header field.
The referrer is the address of the previous web page that the user was
on before arriving at the current page, typically by clicking a link.
Note: When visitors move from page to page within your site, the referrer property
does not contain the address of the page the visitor came from. This is because
Wix sites are built as single page applications.
To get the previous page a 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.