Create JSON Schema spec from Swagger model definitions during docs build

[jaeddy]

See #135 for the origin of this ticket.

Idea (from @susheel) is "to allow a non-swagger client to still validate the JSON response without processing the swagger spec to extract the JSON schema."

Changes in #136 add this JSON Schema file manually, but should be part of the build process: the file doesn't need to be under version control — but we can include a step to process the Swagger/OpenAPI spec and host at a documented path on our GitHub pages site.

[jaeddy]

This issue is partially solved by maintaining schema definitions in (separate files)[https://github.com/ga4gh/workflow-execution-service-schemas/tree/develop/openapi/components/schemas] (converting from YAML to JSON Schema should be a trivial extra step). We'd need to think about whether to also include support for Swagger-specific structures (e.g., definitions instead of components).