server: Rename JudgelsService* to JudgelsApi* and split ServiceUtils

[fushar]

After judgels.service became judgels.core (#1028), a few classes still carried the old "service" name. ServiceUtils also mixed two unrelated things: request checks that nearly every resource calls, and file response builders that only a few call.

Rename the JudgelsService* error classes to JudgelsApi*, and split ServiceUtils in two. No method body, package or JSON field changes.

• JudgelsServiceException → JudgelsApiException and JudgelsServiceError → JudgelsApiError, in judgels.core.api. They are the error an API caller sees: an HTTP status, a message and args.
• JudgelsServiceExceptionMapper → JudgelsApiExceptionMapper, and the integration-test helper JudgelsServiceErrorDecoder → JudgelsApiErrorDecoder.
• ServiceUtils → JudgelsRequestChecks (checkFound, checkAllowed) and JudgelsResponseBuilders (buildDownloadResponse, buildMediaResponse), both in judgels.core.
• Callers change only their imports, plus the qualifier in the 10 resources that call the response builders by class name. Imports are re-sorted in every touched file.

The Gradle module judgels-service-core keeps its name.