CartApproval: Enable admins to edit managers and buyers

[markwoerdenweber]

Confer with Michael about integrating the budget extension first.

Add a new customer tab in the backend.
If the customer is a manager, add a grid of buyers that displays their name, email, budget and cart limits, and has the following functions: add (with and without notification), remove, edit.
If the customer is a buyer, show a list of their managers with a corresponding link.