Allow admins to purchase products for other users

[novoselt]

Potential implementation:

• admins can enter account_id of the customer
• and add either percentage discount or modify the automatically generated price to any value

Then make a choice of the payment method:

• use customer balance
• use customer credit card on file
• complete the purchase inside of CoCalc, but leave the Stripe invoice open with due date in 30 days

Once "confirmed", the purchase generates Stripe invoice with the description of all items, their "regular price", discount, and resulted discounted price.

The CoCalc product resulting from such a purchase should not be refundable/editable by the end user (at least not by default).

The intention is to use it for high-volume customers with whom we may have a personal deal. Or, in case of our prices/poducts changing, we will have an easy way to honor old quotes.