1.0.38: fix: clean up stale files before new import upload

Prevents "File not found" errors caused by stale file references in
storage/app/data-synchronize/. Also adds unit tests for ImportColumn,
Importer, and UploadController.

1.0.37

Translate to some languages

1.0.36

Fix bug

1.0.35

Correct translations

1.0.34

Add default translations

1.0.33

Truncate data when displaying in example

1.0.32

Improve export performance

1.0.31

Update export.blade.php

1.0.30

Save selection to localstorage

1.0.29

Improve code