Refactor cancelling positions and orders in the data model (#1088)

- [x] Data model
- [x] display in order view in backend
- [x] review all usages of OrderPositions.objects
- [x] review all usages of order.positions
- [x] review all other model usages
- [x] review plugins
- [x] plugins backwards-compatible API?
- [x] decide on way forward for REST API
- [x] need to cancel fees
- [x] tests
- [ ] plugins
  - [ ] gdpr
  - [ ] reports
- [x] docs

doc/api/resources/orders.rst

@@ -26,7 +26,6 @@ status                                string                     Order status, o
                                                                  * ``p`` – paid
                                                                  * ``e`` – expired
                                                                  * ``c`` – canceled
-                                                                 * ``r`` – refunded
 secret                                string                     The secret contained in the link sent to the customer
 email                                 string                     The customer email address
 locale                                string                     The locale used for communication with this customer
@@ -58,9 +57,9 @@ invoice_address                       object                     Invoice address
 └ vat_id_validated                    string                     ``True``, if the VAT ID has been validated against the
                                                                  EU VAT service and validation was successful. This only
                                                                  happens in rare cases.
-positions                             list of objects            List of order positions (see below)
-fees                                  list of objects            List of fees included in the order total (i.e.
-                                                                 payment fees)
+positions                             list of objects            List of non-canceled order positions (see below)
+fees                                  list of objects            List of non-canceled fees included in the order total
+                                                                 (i.e. payment fees)
 ├ fee_type                            string                     Type of fee (currently ``payment``, ``passbook``,
                                                                  ``other``)
 ├ value                               money (string)             Fee amount
@@ -874,7 +873,7 @@ Order state operations
 
       {
         "code": "ABC12",
-        "status": "r",
+        "status": "c",
         ...
       }
 
@@ -1066,6 +1065,8 @@ List of all order positions
    The order positions endpoint has been extended by the filter queries ``voucher``, ``voucher__code`` and
    ``pseudonymization_id``.
 
+.. note:: Individually canceled order positions are currently not visible via the API at all.
+
 .. http:get:: /api/v1/organizers/(organizer)/events/(event)/orderpositions/
 
    Returns a list of all order positions within a given event.