Adding basic compatibility with subs table

[dparker1005]

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Changes proposed in this Pull Request:

Adds basic compatibility with subs table by updating subscription data when a check payment is received and not acting on a PBC subscription if the corresponding subscription in subs table has been cancelled.

This PR does not switch the Add On to fully rely on subs table, only bridges the gap for the time being. For full compatibility, see this issue:
#109

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.