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 anon-2562571@example.com from central-digital-and-data-office
Unsubscribing anon-2527485@example.com from central-digital-and-data-office
Subscriber anon-461468@example.com 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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Follow these steps if you are doing a Rails upgrade.

[KludgeKML]

LGMT, excellent work improving coverage of that file as well.