System schema.
Standardize issuer, program models.

Issuers, programs and systems are exactly the structures in the
database, as far as I can tell. This commit standardizes the code that
represents those models.
Add program model test.

It's exactly the same as the issuer model test.
Implement system model.

It's exactly the same as the issuer & program models.
Field sanitizer function.
Add systemId and programId to badge schema.