Allow Cc'ing thread participants that are not in the `sender_domains`

When using Pony Mail to reply to messages from non-subscribers (like for external reports to security@ lists), it would be nice if we could add the original author to the Cc.

This could be an option (default 'off') to allow those addresses in 'Cc'. It would be convenient to show this list of possible recipients in the message compose dialog.

Trace how sender_domains restricts Cc recipients and how the message compose dialog builds its possible-recipient list. Define the default-off option for including non-subscriber thread participants, then verify that those addresses can be shown and selected only when the option permits it.

Written by the indexing model from the issue text.