Relationship reworking

Cleaned them up a bit, and extended to allow relationships between multiple instances of the same class.
Add related_agents
First cut at related agents
Doc update
Schema changes for agent relationships
Add a 'related_agents' mixin that overlays the directional agent relationship on the existing relationships functionality.
Add related agent subforms and scripts for the frontend.
Once we're done processing linked records for a nested record, store that record back in the parent record.

This is necessary because the nested record processing might mutate
the record, and we want those changes in the parent so the real-time
indexing picks them up.
Slot in URI when throwing back an error page to the agent linker
Add solr backups to .gitignore
Limit agent linker only to particular types when creating related agents
Add the date subrecord to agent relationships.
Merge pull request #376 from hudmol/related_agents

Related agents