Don't use referrer as a fallback in VDestination.

Since the referrer is not included in cache keys for pagecache or
Akamai, this can result in weird stuff getting cached and causing
downstream users to get redirected after login/registration to
unexpected destinations.

r2/r2/lib/validator/validator.py

@@ -1795,7 +1795,7 @@ class VDestination(Validator):
 
     def run(self, dest):
         if not dest:
-            dest = request.referer or self.default or "/"
+            dest = self.default or "/"
 
         ld = dest.lower()
         if ld.startswith(('/', 'http://', 'http://')):