Merging the Recurring Emails Add On

[dparker1005]

All Submissions:

• Have you followed the Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Merging the Recurring Emails Add On to notify customers of upcoming payments.

The main change from the original Add On is that, by default, users will only be notified of an upcoming payment if their previous payment was 6 months or more in the past. This modification is useful to comply with the laws of various countries and payment processors without spamming users each time that they have a more frequent payment.

Resolves XXX.

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.

[dparker1005]

The recent commit reversed the "6 month limit" change. I have put together a code recipe here that will add that behavior to sites that want it:
https://gist.github.com/dparker1005/b1ab6f6e47d9611a0c511c71ef1a33ea