This is extracted from #22390.
This patch simplifies a little how we handle uploads in chat, relying on
ActiveRecord mechanisms instead of calling custom methods.
This also makes `Chat::Message#validate_message` a “real” AR validation,
meaning it will run automatically when `#valid?` is called.
Prior to this fix, the upload was removed from DOM when collapsed and not decorated again on expand, which was causing lightbox to not get reapplied. The fix is reverting to previous state where content was not removed from DOM.
