* Change semantics of changing orders
This basically does two things to the "Change products" view of orders and the
OrderChangeManager program API:
1) It decouples changing items or subevents from changing prices.
OrderChangeManager.change_item() and .change_subevent() no longer
touch the price of a position. Instead .change_price() needs to be
called explicitly. However, a client-side JavaScript component now
*proposes* a new price based on the changed item or subevent.
2) The user interface now exposes the possibility of doing multiple
things at the same time, i.e. changing the item, subevent and price
in the same operation. OrderChangeManager already allowed this
before.
(1) is basically a consequence of (2), while (2) is a prerequesite for
e.g. the `seating` branch, where changing the subevent will always
require changing the seat.
* Add tests for price calculation API
* Original price for variations
* Documentation
* API-GET
* Fix existing tests to accomodate new attribute
* Test for variation's original_price on API
* Upgrade django and stuff
* Update to Django 2.2 and recent versions of similar packages
* Provide explicit orderings to all models used in paginated queries
* Resolve naive datetime warnings in test suite
* Deal with deprecation warnings
* Fix sqlparse version
* Data model + Editor
* Cart and order management
* Rebase migrations
* Fix typos, add tests on cart handling
* Add tests for checkout and quotas
* Add API endpoints
* Validation of settings
* Front page tax display
* Voucher handling
* Widget foo
* Show correct net pricing
* Front page tests
* reverse charge foo
* Allow to require bundling
* Fix test failure on postgres
