Enable Series that have multiple authors

[LokiMidgard]

While adding some Books, I stumbled upon a series where every Book has another author.

However when clicking on the series link I only saw one book. While I see why this is implemented in such a way, it will not be able to model such series.

The Series I saw was Justifiers (German link).

[mouse-reeve]

This is related to how series are currently implemented, and would, I believe, be resolved in the same refactor proposed here: #2991