Clicking on template folder navigates to a page that displays that
folder's contents.
This reuses the existing choose template view by adding a filter
based on optional `template_folder_id` argument.
Service model methods are rewritten to match `all_templates` and
`get_template`. New `get_template_folder_path` method returns a
list of folders (from root to the current one) that the selected
folder is nested inside.