Akregator: fix URL/path confusion which led to KIO warnings and possibly missing icons

Summary:
The faviconUrl stored in ~/.local/share/akregator/data was a path rather
than a URL.

It's also pretty pointless to then do a download job on that local file...

Test Plan:
I've been having some trouble finding RSS sites with icons...
Please double-check.

Reviewers: mlaurent

Differential Revision: https://phabricator.kde.org/D28581