✨(models/api) add document model and API

We do this by making copies of existing Template and TemplateAccess
models and API. A little refactoring is done to try to limit duplicate
code.

src/backend/core/tests/templates/test_api_templates_generate_document.py

@@ -107,7 +107,7 @@ def test_api_templates_generate_document_related(via, mock_user_get_teams):
     data = {"body": "# Test markdown body"}
 
     response = client.post(
-        f"/api/v1.0/templates/{access.template.id!s}/generate-document/",
+        f"/api/v1.0/templates/{access.template_id!s}/generate-document/",
         data,
         format="json",
     )

src/backend/core/tests/templates/test_api_templates_list.py

@@ -108,7 +108,7 @@ def test_api_templates_list_pagination(
     client.force_login(user)
 
     template_ids = [
-        str(access.template.id)
+        str(access.template_id)
         for access in factories.UserTemplateAccessFactory.create_batch(3, user=user)
     ]