Follow-up to #14777.
Currently, only if a new entry is added, the book cover is downloaded.

If I open a book where a book cover is available onlíne, it should download it.

• Should happen in the background
• There should be no check for 24 hours if there was no book cover available. The flag could be stored as file named with the isbn but file extension ".not-available" (instead of png)
• Existing functionality for check for existence should be used