text: Title: How to access an Iframe from an external source without uncaught DOMexception?
  Body: I have some other website I own and I want to embed html from that website as Iframes. I want to access some properties of the actual element (such as scroll height) to adjust the Iframe in dash.
  
  But I get a `Uncaught DOMException: Blocked a frame with origin "http://localhost:8050" from accessing a cross-origin frame.`, which is to be expected. In flask there's a way to whitelist other sites, is there a way to do this in Dash?
  
  thank you!