Make the JIRA fields to display configurable

This change allows anyone to define a list of additional fields to display in Morgue - by default, the only reported fields are
 - key
 - summary
 - status
 - assignee
 - link

Additionally, the querying mechansim has been changed to query all the issues related to a post mortem at once (using a JQL query), instead of querying issues one by one.