Add a new method to agents that lets you inspect their roles in linked_agent relationships.

Required a new method on relationships to efficiently access all properties for
a given type of relationship, and a new method on BackendEnumSource to do a
fast bulk lookup of a list of enumeration value IDs.
Added indexing for the new linked_agent_roles field.
Add linked_agent_roles as a search facet to the public searches
When rendering a subject, indicate whether or not it's linked to anything in a 'is_linked' property.  Index that property for easy filtering.
Use a loaded hook to set up reciprocal relationships for subjects and agents.

Saves having to duplicate the knowledge of which models link to agents and subjects.
Add a migration to reindex all subjects and agents
Remove now-obsolete '../migrations' dir from backend's warble.rb
Switch is_linked to be read only in the subject schema
Switch migration to 008 to avoid merge conflict