Improve UI to configure unavailable items handling (Z#23131828) (#3739)

* start impl of unavailability modes ui

* add db migration

* use new widget for more fields

* improve contrast

* use new widget for hide_without_voucher field

* improved wording

* rebase migration

* undo changes to require_membership_hidden

* code formatting

* move unavail_reason logic around

* enforce consistent state of hide_without_voucher / require_voucher

* annotate unavailability info in get_grouped_items

* remove MSIE6 compat

* add unavailability reasons to widget

* remove test output

* Apply suggestions from code review

text improvements

Co-authored-by: Richard Schreiber <schreiber@rami.io>

* add css fix for jumping items due to tooltip

* dynamically retrieve unavailability reason message

* widget: simplify logic conditions

* add available_{from,until}_mode to api and api docs

* rebase migration

* rebase migration

* add unavailable_*_mode to ItemVariation

* add available_*_mode to API docs for items

* fix wrong reference

* fix test cases

* add available_*_mode to item variation form

* apply unavailability modes to subevents and variations (presale)

* /o\

* apply unavailability modes to subevents and variations (widget)

* display unavailability mode in subevent product settings

* fix widget test

* fix api item tests

* copy available_*_mode when copying an item

* Apply suggestions from code review

Co-authored-by: Raphael Michel <michel@rami.io>

* Add unavail mode indicator to bulk create and edit forms

---------

Co-authored-by: Richard Schreiber <schreiber@rami.io>
Co-authored-by: Raphael Michel <michel@rami.io>

src/pretix/static/pretixcontrol/scss/main.scss

@@ -908,6 +908,10 @@ tbody th {
     display: block !important;
 }
 
+.btn-group .btn:not(:first-child) {
+    margin-left: -1px;
+}
+
 @import "../../pretixbase/scss/_rtl.scss";
 @import "../../bootstrap/scss/_rtl.scss";
 @import "_rtl.scss";