Enforce budget on cost estimate validation

This work is intended to bring the enforcement of the budget to the cost estimate validation.

Because cost estimates are... estimates, it is too strict to enforce the budget on the creation (that was disabled already). Instead, we check when the estimate is validated into a logistic order.

- [x] rebase on top of #46 
- [x] check the views
- [x] update the readme
s/required/requires/ ?