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 
I find it quite hard to get my head around this `unless` for some reason. It'll include the private details unless there are none, or the requestor hasn't provided an email address, right?

It might clearer if the mailers provided a template variable along the lines of `@show_private_part` to control this instead. It feels like knowing that the lack of an email address means that private things can't be shown (because the email will then be going to a public email address at WTDK.com) is giving the template a few too many smarts.