Tracks last edited date and user, and adds has_changes for blocks

The `user_id` passed into `_update_single_item()` will always be None, so `edited_by` will always be None, but @nasthagiri will fix that in her work. STUD-1724

@cahrens @andy-armstrong @dmitchell 
A nit: direct only categories are always published-- they can't have drafts. 

Maybe has_changes documentation should say it returns True if the draft and published versions differ?