UX: Hide chat image overflow (#18000)

When there was a problem with an image the built-in frame would expand way outside the message.

app/assets/stylesheets/common/base/onebox.scss

@@ -218,6 +218,7 @@ aside.onebox {
           max-width: initial;
           max-height: initial;
           float: none;
+          overflow: hidden;
         }
       }