Confirm Assignment Payment Workflow

[mhrivnak]

Requirements:

• It should be apparent whether a Task requires payment before the assignment is made, and also how much the payment is.
• An assignment that requires payment should get an initial status of "unpaid", which prevents assignment attempts from being made.
• The status will change to "assigned" once payment is made.
• At the time of status change from "unpaid", we need to store data about who made the change and when.

The Assignment model has a derived property "payment_required", but I think that feature is incomplete. I am also not sure if we want to go through the trouble of using the Product model right now. I'm not opposed to it though if it seems like a reasonable path to take.