Enforce HTTPS links in the "Actions" url field

The  [os2forms_digital_post](https://github.com/OS2Forms/os2forms/tree/master/modules/os2forms_digital_post) handler should enforce that only secure urls with https:// should be accepted in the input field.

A proper human readable notification should be raised that explains the external requirement from Digital Post as a reason.