* rders: Fix N+1 query issues on order detail and change page
There is one I couldn't fix: Loading the list of tax rules for every
select box on the OrderChange page. Unfortunately, Django has a
cache-breaking .all() in ModelChoiceField and that would need nasty
patching that didn't feel worth it
* Fix isort
* Fix N+1 query in gate call
* Fix leftovers
* Add local cache for objects referenced in log entries
* Update src/pretix/control/views/orders.py
