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 in a 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 in a site, you can
use the Storage APIs to store the visitor's current page
and retrieve the visitor's previous page.