Serialize "service" field in /api/hosting-service-accounts/
Serialize "service" field in /api/hosting-service-accounts/

The service field wasn't being serialized correctly, which meant that if you
tried to use this resource to get a list of things, you'd get a 500 instead.

Testing done:
Connected a GitHub repo, then $ curl /api/hosting-service-accounts/

Reviewed at https://reviews.reviewboard.org/r/4702/

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