Make radio form components reusable for nested checkboxes

For the template folders permission editing we need a nested
checkboxes form that is similar to "move folder" input, except
it's using checkboxes instead of radio buttons.

This moves most of the macros into a shared "select-input" components
file, which are wrapped by the existing radios.html by setting the
required input type.

app/main/forms.py

@@ -837,10 +837,7 @@ class RadioFieldWithNoneOption(FieldWithNoneOption, RadioField):
     pass
 
 
-class NestedRadioField(RadioFieldWithNoneOption):
-    def __init__(self, *args, **kwargs):
-        super().__init__(*args, **kwargs)
-
+class NestedFieldMixin:
     def children(self):
         # start map with root option as a single child entry
         child_map = {None: [option for option in self
@@ -864,6 +861,14 @@ class NestedRadioField(RadioFieldWithNoneOption):
         return child_map
 
 
+class NestedRadioField(RadioFieldWithNoneOption, NestedFieldMixin):
+    pass
+
+
+class NestedCheckboxesField(SelectMultipleField, NestedFieldMixin):
+    pass
+
+
 class HiddenFieldWithNoneOption(FieldWithNoneOption, HiddenField):
     pass