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As always, todo list to myself ;)

* Add tests for new `none_ok` behaviour for `List` and `UrlList`.
* Investigate why download of http://pgl.yoyo.org/adservers/ fails
* Implement HTTP redirect in standalone `NetworkManager`.
* Investigate weird segfaults and memory corruption issues - maybe `DownloadItem` gets `deleteLater`ed too early?
* Remove `blocked-hosts` if the setting gets set to the empty string.