- Lists can now be marked as single | double optin.
- Insert subscribers to double opt-in lists send out a
confirmation e-mail to the subscriber with a confirmation link.
- Add `{{ OptinURL }}` to template functions.
This is a breaking change. Adds a new field 'optin' to the lists
table and changes how campaigns behave. Campaigns on double opt-in
lists exclude subscribers who haven't explicitly confirmed subscriptions.
Changes the structure and behaviour of how notification e-mail routines,
including notif email template compilation, notification callbacks for
campaign and bulk import completions.
