Implement Roundup style 'nosy list' for email notification

[Alex Buccino <alexander.buccino@…>]

The Roundup bug tracker's email notification is based around the idea of a 'nosy list'. The nosy list is just the list of email addresses to be notified with each ticket change. Its special characteristic is that a user is automatically added to the list whenever they edit the ticket. Essentially, if you express a stake in a ticket, you are automatically notified for the rest of that ticket's lifecycle.

The nosy list is editable and you can add or remove yourself (or others) manually if desired. Obviously logic needs to be in place so that the act of removing yourself from the nosy list is an exception to the rule that editing a ticket adds you to the nosy list.

[cboos]

The nosy list model could indeed be used to
redesign the CC field (#1459).