Role facets for agents

Add some new Solr document fields for agents and subjects:

  * For agents, linked_agent_roles gives a list of the roles that each agent has been linked with (e.g. "creator" or "source").  The "is_linked" field is a boolean that indicates whether an agent has been linked to a record.

  * For subjects, the "is_linked" field indicates whether a subject has been linked to a record.