* previously, this property was missing from the fake entity which would
  trip up notification builders

* setting an empty value in the settings form previously caused a
  validation error because the empty string is not a valid email address

* this brings it more in line with the request notifications
* previously we were restricted to emails, now it will be whatever
  notification backends we use
* also remove the mail templates - they now live in code until we
  enhance the notification backend to be more flexible w.r.t. the
  rendered templates