Correction options refactor

[samlown]

• Removing the "Credit" and "Debit" booleans from Correction options and instead adding the type field.
• Standardising on a standard approach for dealing with corrective documents for each type:
  • corrective invoices completely replace the previous invoice, which should now be disregarded.
  • credit-note for partial or complete refunds.
  • debit-note is the least used, but reflects additional charges.