fixes https://github.com/TryGhost/Product/issues/2947
When an import is performed and a member already exists:
- If `subscribe_to_emails` is set to `true` and the existing member
already has newsletter subscriptions, these are preserved
- If `subscribe_to_emails` is set to `true` and the existing member does
not already have newsletter subscriptions, no new subscriptions are
added in case the member has previously unsubscribed
- If `subscribe_to_emails` is set to `false` the existing member will be
unsubscribed from all newsletters regardless of their preference
b49cf14287