Reviewed and refactored additional editor draft save warnings

Reviewed and refactored additional editor draft save warnings

- Added testing to cover warning cases.
- Refactored logic to be simpler and move much of the business out of
  the controller.
- Added new message that's more suitable to the case this was handling.
- For detecting an outdated draft, checked the draft created_at time
  instead of updated_at to better fit the scenario being checked.
- Updated some method types to align with those potentially being used
  in the logic of the code.
- Added a cache of shown messages on the front-end to prevent them
  re-showing on every save during the session, even if dismissed.