Add edit_folder_permissions permission

Admin part of alphagov/notifications-api#2356
Adds Folder permissions switch to the platform admin part of service
settings page.

tests/app/main/views/service_settings/test_service_setting_permissions.py

@@ -46,6 +46,8 @@ def test_service_set_permission_requires_platform_admin(
     ('inbound_sms', 'False', False),
     ('email_auth', 'True', True),
     ('email_auth', 'False', False),
+    ('edit_folder_permissions', 'True', True),
+    ('edit_folder_permissions', 'False', False),
 ])
 def test_service_set_permission(
     mocker,