Add support for subsumes operation for non-SNOMED code systems and across CodeSystem boundaries.

[wardle]

The subsumes endpoint supports subsumption testing for SNOMED CT. We can use relationships in other code systems to calculate subsumption (e.g. naive subsumption using Read codes or ICD10 based on their hierarchies), but we can also provide subsumption testing across CodeSystem boundaries. Indeed, the naive ICD-10/Read subsumption would be less preferable than one enabled by cross-mapping.