[FIX] mrp_subcontracting: extra move correctly handle.

Usecase to reproduce:
- Create a receipt of 1 units of a subcontracted product
- Receive 2
- Validate the receipt

-> An extra move for subcontractor of 1 has been validated but a
new move of 2 still in waiting availability and can not be
validated.

It happens due to action_done that will validate existing subcontract
order (with the initial quantity). Then the move is validated and an
extra move is created and merged in the first one. This new merged
move of 2 is then confirmed and validated. That will recreate a
subcontract order of 2 that will not be validated since it was validated
in the picking action_done at the start.

This commit will modify:
- action_done on move in order to validate the subcontract order after
the move validation and the new subcontract order creation too.
- action_confirm in order to create the subcontract order with the
additional quantity and then merge it to the existing moves.