Provide credit purchase method for pro users and org admins

[allanlasser]

Reviewing #311, Mitch and I decided to punt on credit purchasing for now.

To purchase credits, we should show a dialog that lets you pick the # of credits you want to purchase and provides a credit card input. We can probably use Square Checkout for the credit card part of this, which is how users purchase requests on MuckRock.

Purchasing credits also requires API updates that will be sent the order and the credit card authorized payment token from Checkout.js, then update the organization's account balance.

UI related to purchased credits is present in the codebase as part of #311, but commented out with TODO flags and tagged with this issue #.