Add manual invoicing

Based on #1 (closed), add a possibility to create a manual invoice from the invoice group detail view:

• Amend the Invoice model with:
  • a reference to a Person
  • a reference to invoice items stored in the database
    • make get_purchased_items yield these items if set
  • a constraint to allow only one of person or for_object being set
• Add UI for creating such a manual invoice from the invoice group detail page
  • Fields for invoic edetails, including billing address etc.
  • Field to select a person
    • If person is selected, auto-fill its details into the billing information
  • Table to list invoic eitems
    • Ideally, this should use a form set, so one can really enter the items into the table