Sale Exception Credit Limit

NOTE: this module replace partner_credit_limit that is going to be depreceated on v9. It adds a new group "can modify credit limit", only users with this group are allowed to change credit limit on partners.

It also adds an exception to check that you can not aproove sale orders that would exceed credit limit. It checks:

• The credit the Partner has to paid
• The amount of Sale Orders aproved but not yet invoiced
• The invoices that are in draft state
• The amount of the Sale Order to be aproved

and compares it with the credit limit of the partner. If the credit limit is less it does not allow to approve the Sale Order