Add post-contributor route

Add post-contributor route

• Register route
• Add callback to fetch data

data will be returned in the form:

{
   'author name': [ 
      'author', 
      'interviewer', 
      'translator'
   ],
   'author name': [ 
      'interviewer', 
   ],
   'author name': [ 
      'translator'
   ],
}

Add display_name property for consistency
Clean up method formatting
Return translator object instead of id
Suppress Shortcode alert message