Void an unpaid invoice when plan is canceled

[allanlasser]

If a plan renews with an invoice payment method, an invoice is issued. If a customer doesn't wish to renew, we cancel their plan. When the plan is canceled, any unpaid invoices should also be voided.

[amandabee]

When @morisy and I looked at this he flagged that that may not always be the case -- because in some cases someone has used the service while the invoice was open. I haven't fully thought this through but wanted to flag the potential for outlier cases, especially wrt the period between enrolling and canceling, or renewing and canceling.