Clarify the related agents code by naming the date handling method.
Generalise related agents code to let us model relationships that have directions.
Change the way we store relationship directions so it works for non-global records too.

Previous we would store 'relationship_target', which was the URI of the
record that a relationship pointed to.  This worked fine for global
records, but could be troublesome for repository-level records, as these
can have different URIs over their lifetime if transferred between
repositories.

Now we store a pair instead: the record type and its ID.  This should be
stable throughout a record's lifetime.
Change the new directional relationship API to be a drop-in replacement for define_relationship
Add backend implementation of hasPart/isPartOf
Add support for the bound_with relationship too
Switch both relationship types to use the same JSON property
Frontend changes to support the new types of accession interrelationships.
Make sure related accessions don't interfere with spawning resources.
Code review suggestion: use the display string when showing related accessions in the read-only view.
Add a new frontend test to catch missing dynamic enumeration translations
Added missing I18N values for new accession relator types
Fix the 'Related accessions' anchor in read-only view
Generalise 'bound' relationship to be 'sibling'.
Switch dynamic enums to support properties taking values from more than one enum
Add a new 'relator_type' property to accession relationships.
Merge pull request #772 from hudmol/accession-to-accession-relationships

Accession to accession relationships