Mmpu compatibility

[dparker1005]

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Changes proposed in this Pull Request:

Add compatibility with PMPro Multiple Memberships Per User. Note that this does not include compatibility for purchasing multiple levels in a single checkout, only for managing users who have multiple levels.

Builds off of #56.

Specific changes made include:

• Considering all of a user's level when generating a link to checkout in pmproap_getLevelIDForCheckoutLink() function
• Now keeping correct enddate after purchasing an add-on package instead of choosing the enddate for the first level that the user has

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.