One user could edit the draft and save it while the other one is changing the preview of the same x-page in the channel manager. We do not want this kind of concurrent modification by different users of the same x-page.

UI Experience

If another user is editing the document (aka is editing the draft and is the holder) we'll show the XPage in the Channel Manager as being locked with the userId of the user holding the lock. 

See attached image or: https://bloomreach.invisionapp.com/share/N4WCV1BEUXC#/screens/417107899_0481_Locked_-content_Is_Edited_By_Other_User- for design: banner, overlay (locked containers, read-only components) and menu