Use Select2 for subevent and other long selections (#763)

* Use Select2 for subevent and other long selections

* Minor correction

src/pretix/control/forms/widgets.py

@@ -0,0 +1,38 @@
+from django import forms
+
+
+class Select2Mixin:
+    template_name = 'pretixcontrol/select2_widget.html'
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+    def options(self, name, value, attrs=None):
+        if value and value[0]:
+            for i, selected in enumerate(self.choices.queryset.filter(pk__in=value)):
+                yield self.create_option(
+                    None,
+                    self.choices.field.prepare_value(selected),
+                    self.choices.field.label_from_instance(selected),
+                    True,
+                    i,
+                    subindex=None,
+                    attrs=attrs
+                )
+        return
+
+    def optgroups(self, name, value, attrs=None):
+        if value:
+            return [
+                (None, [c], i)
+                for i, c in enumerate(self.options(name, value, attrs))
+            ]
+        return
+
+
+class Select2(Select2Mixin, forms.Select):
+    pass
+
+
+class Select2Multiple(Select2Mixin, forms.SelectMultiple):
+    pass