Consider `OstreeRef`, `OstreeConfig`, and other models artifactless

[lubosmj]

It is not necessary to store the checksum of a commit or a couple of lines for a configuration in a separate file. Use TextField instead.

[lubosmj]

This could improve the speed of syncing and importing. Besides that, we will also be able to perform various operations inside repositories more efficiently without accessing the storage backend.

The problem might lie in setting relative paths of remote artifacts. This was the main reason for introducing the models with artifacts.