Improve the logging during image import (see #11020)

This PR extends the contents of the import log file created during the upload/import phase. The file can be accessed after the import finishes in Insight (using the ``View`` button).

The ``logback.xml`` configuration modifications might slightly affect ``Blitz-0.log`` and friends (after local comparison, the contents of those log files should be roughly the same before this PR and after).

To test - upload images (different types) and view the import log file - contents should be quite verbose (messages of levels: DEBUG, INFO, WARN and ERROR - latter two not always present).