Invoices:
- the user should be able to create/edit/delete invoices
- fields:
  - employee (usually the user who created the invoice. Can be edited by admin/superuser)
  - customer (mandatory)
  - contact person (picked from the available list of contact persons of the customer)
  - date
  - due date (auto-calculated as date + payment term no. of days), can be edited
  - name (loaded from customer, editable)
  - address (loaded from customer, editable)
  - project
  - cost center
  - currency
  - credit invoice yes/no
  - amount (auto-calculated as sum of net amount of all invoice rows)
  - rebate type (absolute / percentage)
  - rebate value
  - net amount (auto-calculated as amount - rebate)
  - vat (auto-calculated as net amount * <vat rate>)
  - gross amount (auto-calculated as net amount + vat)
  - payment term (linked to payment terms table)
  - vat (linked to vat table)
  - reverse charge yes/no
  - note field (multiline)
  - OCR field (string field)
- invoice rows:
  - can be loaded from an expense, a project report, a supplier invoice or an article. If a project has been selected in the invoice header, the available items will be filtered by the given project
  - fields:
    - name
    - description
    - quantity (only editable if article selected)
    - amount (extracted from the selected linked item)
    - rebate type (absolute / percentage)
    - rebate value
    - net amount (auto-calculated as amount - rebate)
- remarks:
  - multiple expenses, project reports, etc. could be optionally cumulated into a single invoice row (e.g. we have 30 invoices but we invoice only the total, no need to display details)
  - all linked items will get the "invoiced" flag (either by setting a bool value on the table or by computing it based on the link), meaning they can no longer be selected in another invoice
  - possibility to export as PDF and should be able to send to the invoicing contact person of the customer and optionally to the invoice contact person as cc

Payment Terms:
- fields: name, no. of days
- optionally linked to a customer, else global
- default added for every company: 15, 30, 45 days

VAT:
- fields: amount
- optionally linked to a customer, else global
- defaults: 0%, 6%, 12%, 25%

Customers:
- add contact persons secondary table which contains (for now) the following information: First Name, Last Name, Email Address
- add invoicing settings:
  - optional default payment term
  - optional default vat
  - optional default contact person

Articles:
- Usually denotes stuff that is invoiced periodically to a customer but does not fall in one of the existing categories (expenses, project reports, supplier invoices)
- fields: name, description, active date range (?)
- optionally linked to a customer (maybe a company invoices the same thing to all of its customers)
- includes price