Wishlist: Implement budgets

Some teams have budget allocations (for example, Debian Montréal files for a budget on an annual budget, Debian Social has a monthly approved budget for hosting).

It would be great if the system could be aware of team budgets so that each expense doesn't need an explicit approval, and so that we can tell what percentage of team budgets are used, and the system could potentially be the canonical place for approving/managing such budgets.