Mark budgets and contracts as closed

[tinne]

Budgets and Contracts should have an optional end date, after which all hours imported for the Budget (any Budget of the Contract, resp.) are marked as erroneous.

If a Contract has and end date, none of its Budgets may end later, and the Budget creation / edit views shall inform about the Contract end date.

The Budget or Contract end date may be later than the project end date.

In a report, a budget or contract should be displayed as already closed if the report time frame ends before or at its end date. The budget and contract reports should report on already closed budgets separately, that is, there should be one table for not-already-closed and another for not-already-closed budgets.