Adding a thumbnail does not show in parent

## Steps
Given structure A > B, where B does not have a thumbnail

- Change B's thumbnail
- Go in A

Expected behavior: B should have its thumbnail displayed
Current behavior: B does not have a thumbnail

The cause if the `children` key, that contains packed items (with thumbnail info). This info is not invalidated when a thumbnail is uploaded.
It works if the item already has a thumbnail, because the link references the same path (that is updated by the server)