Support double opt-in in Webform handler

Note that the webform handler only works after applying the patch from 🐛 Webform Handler does not work RTBC first.

To avoid merge conflicts when including both this patch and the one from 🐛 Webform Handler does not work RTBC , I also included the fix from there.

Thanks @mrshowerman, I'll review it.

Rebased on 1.2.0.