CB Paid Subscriptions

This should allow changing plan options mid-cycle of a subscription with automatic prorating. The way this would work is it would calculate remaining value for prorating similar to how plan upgrades work. Next it would go through basket and payment process normally. Once that completes it needs to internally cancel the previous subscription (recurring must cancel). This would be a NEW subscription NOT a modification of existing subscription for most reliability.

Potential complications are primarily with triggers and user state changes during this internal cancellation.