CORE-5832 Added /app/:id/ui response model schema.

Added an App JSON model schema for the edit App endpoint.
Added metadactyl.util.service/swagger-response, since including a schema model as a :return parameter in a swaggered route requires just a clojure map in the response body, which is automatically encoded by the compojure-api libraries.
Removed the App parameter ordering from the metadactyl.zoidberg query, since the ordering field is for command-line ordering and not UI order.
Refactored metadactyl.persistence.app-metadata.relabel/remove-nil-vals into metadactyl.util.conversions, since this endpoint will return an error for nil values in the response map with the current model schema.