Authority import fixes

Some changes here in support of https://www.pivotaltracker.com/story/show/62026654:

  * I've modified the MARC import definitions to add unauthorized names to agents when creating them.  While creating an agent record from a 1XX field, we now check for a set of corresponding 4XX fields and add those as names too.

  * I've added subject field definitions for authority records.  We'll pull in subjects from the 15X fields, as well as any unauthorized subject forms listed in the corresponding 45X fields.

  *  If you tried importing an empty set of records, the job handler gave a pretty useless JSON parsing error message.  I've added a bit of logic to catch the case where you're trying to import an empty set.