Now taking advantage of subscriptions table

[dparker1005]

In PMPro 3.0, we are adding the pmpro_subscriptions table which will track subscriptions and next payment dates more accurately.

This PR has the following benefits for PMPro 3.0 sites:

• Compatibility with plugins/custom code that modify next payment dates
• Simplified database query
• Simplified tracking for renewals that have already been sent
• MMPU compatibility
• Backwards compatibility when migrating from pre-3.0 setups

In order to implement those improvements, the following features have also been deprecated:

• !!billinginfo!! email template variable, which may not have been accurate previously anyways
• The $lastorder parameter of the pmprorm_send_reminder_to_user filter since we are no longer querying by orders