Add inkstep_conversion_services gem and xsweet Docker service

* Add "xsweet" to docker-compose.yml, for the XSweet docx-to-html web service.
* Add inkstep_conversion_services to StemGemfile, to make the conversion step available in INK.
Status Name Job ID Coverage
failed test #5659

Name Stage Failure
test Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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