Refactor ee and non-ee api_spec.js

Updates api_spec.js (and its ee/ counterpart) to use status codes from
http_status.js and refactors away some duplicate boilerplate.

app/assets/javascripts/lib/utils/http_status.js

@@ -22,6 +22,7 @@ const httpStatusCodes = {
   CONFLICT: 409,
   GONE: 410,
   UNPROCESSABLE_ENTITY: 422,
+  INTERNAL_SERVER_ERROR: 500,
   SERVICE_UNAVAILABLE: 503,
 };