Require a worker version ID when enabling Transkribus in the YAML config

It is currently possible to enable Transkribus imports without configuring a worker version ID. The import task can handle this and will just create elements without a version and not send any transcriptions, which makes no sense. On all of our instances (and probably on dev setups too), we either have Transkribus off, or on with a worker version ID. We can make the version ID required in the config parser.