Add 'Answer References' API endpoint

[alexander-elgin]

1. It shouldn't require authentication
2. By the assessment id e.g. US-2016-HY it should return a list of answers
3. Each answer item should include the linked question data
   1. text
   2. order
   3. precept
      1. title
      2. order
4. Each reference data should include the following data
   1. S3 URL
   2. Title
   3. Other document metadata e.g. publisher, year of publication, authors and etc.