Support deprecated pricing on account page

https://trello.com/c/Cl6H6nIU/378-update-pricing

We introduced new pricing which didn't display correctly on the account page. This change fixes the issues.
It took me a bit to figure out what `MontlyLineItem` could be. `LineItem` isn't really descriptive, unless I'm an accountant. How about something more descriptive like `MonthlyCharge` or `MonthlyRepoCharge`?