podcast: make album art loading work for episodes with different authors

There are some cases where some episodes in a feed can have different authors compared to the feed author. E.g. the participants in an episode are sometimes featured as authors. In other cases, the feed author might have changed the official name over time. In all these special cases, we set the feed author as album artist in all episodes which have different author from the feed.

This should make album art work without any extra code.