Fix subscriptions being immediately cancelled

[dparker1005]

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Changes proposed in this Pull Request:

For core gateways, if the user has a membership level and purchases it again with a subscription, the new subscription will be immediately canceled. This is happening because:

1. The order was being saved in success status before the level changed occurred
2. This caused the PMPro_Subscription entry to be added before level change
3. During level change, all existing subscriptions for cancelled levels are deleted

The fix: Delay the saving of the success order (and therefore the creation of the PMPro Subscription) until after the level change and subs cancellations have been completed.

Other information:

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Changelog entry

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