Add rake task to bulk unsubscribe active subscribers from a subscription list

[deborahchua]

When organisations close/merge, we want to be able to unsubscribe all active subscribers from the organisation's subscription list. This rake task makes it easier to do this in bulk rather than individually using the support:unsubscribe_single_subscription] rake task.

This has been tested on Integration.

Sample output:

Unsubscribing [email protected] from central-digital-and-data-office
Unsubscribing [email protected] from central-digital-and-data-office
Subscriber [email protected] already unsubscribed from central-digital-and-data-office

Trello card: https://trello.com/c/jU0FVxFn/443-bulk-emailing-email-subscribers-of-closing-organisations

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