Remove System.Drawing dependency

In this release we removed System.Drawing dependency from most of our projects so that preview generation may run in Linux containers with .Net 6.

What's Changed

• Prepare preview with Aspose for containerization by @VargaJoe in #7
• SNaaS and standalone projects in the same branch by @tusmester in #9
• Graylog settings placeholder for production container environments by @VargaJoe in #11
• Convert image documents using Skia. by @tusmester in #12

Full Changelog: di-and-logging...remove-systemdrawing