Align on a SSOT for the duo todo body between frontend and backend

The following discussion from !192386 (merged) should be addressed:

• @dstull started a discussion: (+1 comment)

  note (non-blocking):

  body here is correct as it matches the message displayed in frontend. See https://gitlab.com/gitlab-org/gitlab/blob/545208-todo-for-duo/app/assets/javascripts/todos/components/todo_item_body.vue#L166-166

  In a follow-up, we should explore more how to make this a SSOT so we don't repeat on the frontend and cause possible drift. Placing here in this module with that in mind.