Default due date settings

[josemi-ca]

Implement settings to set a default due date.

This could also allow to optionally change default due date depending on the amount. For example: <100€ invoice -> 30 days / 100-999€ invoice -> 15 days / +1000€ invoice -> 7 days

Discussed on commit fd130d2bf6dcc49133f2e9004e53a234576baf0c