Adding MMPU compatibility

[dparker1005]

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• [x] Have you followed the Contributing guidelines?
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Changes proposed in this Pull Request:

Making sure to get the membership level that was actually purchased in pmpro_affiliates_generate_affiliate_after_checkout, and simplifying pmpro_affiliates_no_order_checkout to avoid making excess orders that are potentially for a different level than the one purchased.

Other information:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.