Placeholders in subject were not being replaced because there was
no `.format()` called on the subject.
This commit creates a context dict that is used for both the body
and the subject. It is then replaced using `.format_map()`
Fixes https://github.com/pretix/pretix/issues/531
* New concept for fee handling
* More usages
* Remove all usages, make all tests pass
* API changes
* Small fixes
* Fix order of invoice lines
* Rebase migration
* Data model
* little crud
* SubEventItemForm etc
* Drop SubEventItem.active, quota editor
* Fix failing tests
* First frontend stuff
* Addons form stuff
* Quota calculation
* net price display on EventIndex
* Add tests, solve some bugs
* Correct quota selection in more places, consolidate pricing logic
* Fix failing quota tests
* Fix TypeError
* Add tests for checkout
* Fixed a bug in QuotaForm
* Prevent immutable cart if a quota was removed from an item
* Add tests for pricing
* Handle waiting list
* Filter in check-in list
* Fixed import lost in rebase
* Fix waiting list widget
* Voucher management
* Voucher redemption
* Fix broken tests
* Add subevents to OrderChangeManager
* Create a subevent during event creation
* Fix bulk voucher creation
* Introduce subevent.active
* Copy from for subevents
* Show active in list
* ICal download for subevents
* Check start and end of presale
* Failing tests / show cart logic
* Test
* Rebase migrations
* REST API integration of sub-events
* Integrate quota calculation into the traditional quota form
* Make subevent argument to add_position optional
* Log-display foo
* pretixdroid and subevents
* Filter by subevent
* Add more tests
* Some mor tests
* Rebase fixes
* More tests
* Relative dates
* Restrict selection in relative datetime widgets
* Filter subevent list
* Re-label has_subevents
* Rebase fixes, subevents in calendar view
* Performance and caching issues
* Refactor calendar templates
* Permission tests
* Calendar fixes and month selection
* subevent selection
* Rename subevents to dates
* Add tests for calendar views
* added signature field -- no function yet
* added mail signature feature
* fixed style issue
* fixed problem with signature default
* added unit test for mail signatures
* added unit test for mail signatures
* New models
* CRUD UI
* UI for adding/removing team members
* Log display for teams
* Fix invitations, move frontend
* Drop old models (incomplete)
* Drop more old stuff
* Drop even more old stuff
* Fix tests
* Fix permission test
* flake8 fix
* Add tests fore the new code
* Rebase migrations
* add basic instrumentation possibilities to pretix
* make tabs to spaces
* apply flake8
* implement upstreams suggestions, round 1
* adjust naming of redis-connection
* address noredis
* add view for metrics
* implement HTTP basic auth in front of metrics-endpoint
* rename labelset
* make flake8-clean
* implement upstreams suggestions, round 2
* correct minor slipups
* fix missing return
* let isort add an empty line
* implement test for counter
* implement upstream suggestions, round 3
* correct typo
* implement first test for view
* finish view-test
* fix deprecated keyword
* implement upstream-suggestions, round 4
* implement test for gauge
* test exceptions as well
* add db-decorator
* Check that a voucher's variation matches its item
* Add method to check applicability of a voucher
* Add method to check if a voucher can be used
* Add tests for new voucher methods
* Test for ValidationErrors in Voucher.clean()
* Test for voucher state during ordering process
