Improve the "refer a friend" page

Here are some suggestions for improving the refer a friend page: * Make it a page with its own URL (rather than simply a popup everywhere) so that we can link it in emails, buttons, and QR codes. * Modernize and update the popup itself, it looks as though it was one of the first things that was built for Sprwt and has not received much updating to match the rest of the software since. * Allow us to change the photo on the page Attached is an image of the current, as well as a custom URL page we created ourselves on mailerlite (which unfortunately doec not link to client's accounts, only sends out a promo code)