Display nested folders permissions form on user permissions page

We're reusing the logic for the `move_to` nested radios field for the
user folder permissions nested checkboxes.

The main difference between the two forms (aside from the different
input type) is that "Move" form contains the root "Templates" as an
option, whereas the folder permissions doesn't.

It turns out that, because of the way NestedFieldMixin.children and
select_nested macro are implemented the easiest way to get the desired
folder permissions behaviour is to add the root folder as a choice with
a `None` value and `NONE_OPTION_VALUE = None` set on the field, which
allows the `child_map` to be constructed but doesn't display the root
folder checkbox itself since it gets overwritten in the final `child_map`.

app/main/forms.py

@@ -418,6 +418,15 @@ PermissionsAbstract = type("PermissionsAbstract", (StripWhitespaceForm,), {
 
 
 class PermissionsForm(PermissionsAbstract):
+    def __init__(self, all_template_folders=None, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        if all_template_folders:
+            self.folder_permissions.all_template_folders = all_template_folders
+            self.folder_permissions.choices = [
+                (item['id'], item['name']) for item in ([{'name': 'Templates', 'id': None}] + all_template_folders)
+            ]
+
+    folder_permissions = NestedCheckboxesField('Folder permissions')
 
     login_authentication = RadioField(
         'Sign in using',
@@ -437,8 +446,9 @@ class PermissionsForm(PermissionsAbstract):
         return (getattr(self, permission) for permission, _ in permissions)
 
     @classmethod
-    def from_user(cls, user, service_id):
+    def from_user(cls, user, service_id, **kwargs):
         return cls(
+            **kwargs,
             **{
                 role: user.has_permission_for_service(service_id, role)
                 for role in roles.keys()