Billing plan change business logic

[sebdeckers]

When a new plan is chosen:

1. Cancel current plan
2. Refund amount of current plan, pro-rated by number of days remaining in the billing cycle (year/month)
3. Activate the new plan: charge full amount and start billing cycle on the new date

[alyssaq]

1. Cancellation supported by changing the plan to open source.
2. TODO: Refund remaining amount when user cancels or switches plan.
3. Re-creation of a new plan supported.