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.
Remove old table.
Implement system routes.
Fix test data.
Return ID when getting badge data.
Make sure descriptions get saved.
Merge pull request #21 from mozilla/14-systems

Systems model & API.

[closes #14]