Handle public response emails differently from public ones

* Prefer a direct email address (if given) rather than the public remote email address for sending replies
* Don't include private detail in emails sent to a public address
* Restrict the `private_part` response field in the admin screen if there isn't a suitable email address to send private information

Closes #218 
This is also a lesser case of the above I think - perhaps the controller could set another simpler boolean here instead, like `@can_ask_for_feedback` so we can move this logic and knowledge of the underlying models up a level (where we can comment it to explain *why* the lack of an email means we can't send feedback)