Add simple get_model() on backend
Add endpoint to update the definition only
Update changelog
Revert "Add simple get_model() on backend"

This reverts commit 4788cb262363146f20fa88ec1929f89f54e4557e.
Rewrite definition put without backend get_model()
Fix failing tests
Merge pull request #257 from spiral-project/231_save_model_via_definition

Create/update model via definition endpoint (fixes #231)