Implement import (+ export?)

[tothlp]

Based on the deprecated file-based listing

[tothlp]

As moving the storage to database (#2 ) was a breaking change, we should introduce a TIP message in the list subcommand, if no entries exist, suggesting that maybe "you should try importing, maybe you have records in your json"

[tothlp]

Also, pay attention for duplicates in migration, log number of successfully migrated entries, etc.