Added 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.

Specific changes made include:

• Checking all levels to see if they are pending in pmpropbc_isMemberPending() instead of just choosing one
• Deprecating misspelled pmprobpc_isMemberPending() function
• Now passing post IDs to pmprobpc_memberHasAccessWithAnyLevel() when checking if a user has access to specific restricted content
• Now listing all levels with pending payments on Account page
• Making sure Confirmation message is for the correct level
• Making sure emails have level data for the correct level

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• 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.