Reuse TemplateList class when deleting a folder

Part of moving "get_template_folders" et al. into TemplateList so we
can cache it more effectively. This is slightly less efficient as
iterating a TemplateList will instantiate an object for each item
in the folder; but the difference is minimal.

Note that:

- The default template_type for TemplateList is "all".
- We need to pass realistic template "JSON" in the test now.

app/main/views/templates.py

@@ -470,10 +470,9 @@ def manage_template_folder(service_id, template_folder_id):
 @user_has_permissions('manage_templates')
 def delete_template_folder(service_id, template_folder_id):
     template_folder = current_service.get_template_folder_with_user_permission_or_403(template_folder_id, current_user)
+    template_list = TemplateList(service=current_service, template_folder_id=template_folder_id)
 
-    if len(current_service.get_template_folders_and_templates(
-        template_type="all", template_folder_id=template_folder_id
-    )) > 0:
+    if not template_list.folder_is_empty:
         flash("You must empty this folder before you can delete it", 'info')
         return redirect(
             url_for(

app/models/service.py

@@ -650,12 +650,6 @@ class Service(JSONModel, SortByNameMixin):
             template,
         ]
 
-    def get_template_folders_and_templates(self, template_type, template_folder_id):
-        return (
-            self.get_templates(template_type, template_folder_id)
-            + self.get_template_folders(template_type, template_folder_id)
-        )
-
     @property
     def count_of_templates_and_folders(self):
         return len(self.all_templates + self.all_template_folders)

tests/app/main/views/test_template_folders.py

@@ -5,7 +5,7 @@ from flask import abort, url_for
 from notifications_python_client.errors import HTTPError
 
 from app.models.user import User
-from tests import sample_uuid
+from tests import sample_uuid, template_json
 from tests.conftest import (
     SERVICE_ONE_ID,
     TEMPLATE_ONE_ID,
@@ -968,7 +968,7 @@ def test_delete_template_folder_should_detect_non_empty_folder_on_get(
     ]
     mocker.patch(
         'app.models.service.Service.get_templates',
-        return_value=[{'id': template_id, 'name': 'template'}],
+        return_value=[template_json(service_one['id'], template_id)],
     )
     client_request.get(
         'main.delete_template_folder', service_id=service_one['id'],