allow admin to enter annotations to "Invoices and receipts" for a customer

[DrXyzzy]

Allow admin to enter an annotation to the list displayed under Account / Subscriptions and Course Packages / Invoices and receipts, so that the customer has a more complete view of their account. This could be an addition to the Admin tab. Use cases where a manual entry would be informative:

1. Payment is received that does not go through Stripe.
2. Payment is received via Stripe invoice that is not for immediate purchase of a listed product, e.g. for a subscription that starts in the future.
3. Explanation when adjusting plans & billing cycle, e.g. customer buys a new subscription when intending to extend an existing plan.