title: Insert a UIViewController in an existing UINavigationController
body: <p>I'm developing an app (in Swift with UI builder) where I have a UITableViewController embedded in a UINavigationController so I can present details about the rows, use a standard '+' button to add rows to the table etc. At that point, the UINavigationController is the entry point to the storyboard. </p>
  
  <p>Now I want to add a login screen as a UIViewController before the UITableViewController in the storyboard, so the user needs to log in first and, upon successful authentication, would be taken to the table view. I'm expecting the new view controller should be embedded within the existing navigation controller (makes sense to me for the sake of continuity). However, I can't figure out how I can insert the login screen within the navigation controller without breaking the existing navigation. I've tried several approaches, e.g. creating a new segue from the login screen to the table view, then replacing the relationship "root view controller" between the navigation controller and the table view with a new one with the login screen, but when I do this all the navigation elements in the table view and the subsequent screens (details, data creation) disappear. </p>
  
  <p>Is there a trick to doing this, or will I have to recreate the whole navigation logic? </p>
  
  <p>Thanks!</p>