Make creating TemplateListServices consistent
Make creating TemplateListServices consistent

TemplateListServices are used when we want to show the service
as an additional layer of hierarchy when a user copies a template,
potentially across services [^1].

Normally a TemplateFolder is given "folders" and "templates" by
TemplateList [^2]; TemplateListService was doing it the other
way round and getting its own instead.

Using a subclass of TemplateList means we can make the approach
consistent, which will support the caching approach later on, as
well as simplifying how we work with templates and folders.

[^1]: https://github.com/alphagov/notifications-admin/blob/2e637f801f1f1ebfe1ff19fba7e19bbf40b7cfb4/app/main/views/templates.py#L356
[^2]: https://github.com/alphagov/notifications-admin/blob/bef0382cca16ba87d04430f8cd391092b1dc91ef/app/models/template_list.py#L31-L36

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin