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`.

tests/app/main/views/test_manage_users.py

@@ -228,6 +228,7 @@ def test_service_with_no_email_auth_hides_auth_type_options(
     auth_options_hidden,
     service_one,
     mock_get_users_by_service,
+    mock_get_template_folders
 ):
     if service_has_email_auth:
         service_one['permissions'].append('email_auth')
@@ -249,6 +250,7 @@ def test_service_with_no_email_auth_hides_auth_type_options(
 def test_service_without_caseworking_doesnt_show_admin_vs_caseworker(
     client_request,
     mock_get_users_by_service,
+    mock_get_template_folders,
     endpoint,
     service_has_caseworking,
     extra_args,
@@ -304,6 +306,7 @@ def test_manage_users_page_shows_member_auth_type_if_service_has_email_auth_acti
 def test_user_with_no_mobile_number_cant_be_set_to_sms_auth(
     client_request,
     mock_get_users_by_service,
+    mock_get_template_folders,
     user,
     sms_option_disabled,
     expected_label,
@@ -353,6 +356,7 @@ def test_user_with_no_mobile_number_cant_be_set_to_sms_auth(
 def test_should_show_page_for_one_user(
     client_request,
     mock_get_users_by_service,
+    mock_get_template_folders,
     endpoint,
     extra_args,
     expected_checkboxes,
@@ -419,6 +423,7 @@ def test_edit_user_permissions(
     mock_get_users_by_service,
     mock_get_invites_for_service,
     mock_set_user_permissions,
+    mock_get_template_folders,
     fake_uuid,
     submitted_permissions,
     permissions_sent_to_api,
@@ -474,7 +479,8 @@ def test_edit_user_permissions_including_authentication_with_email_auth_service(
     mock_set_user_permissions,
     mock_update_user_attribute,
     service_one,
-    auth_type
+    auth_type,
+    mock_get_template_folders
 ):
     service_one['permissions'].append('email_auth')