Materials can only have one author

[ocdtrekkie]

When I first wrote this in ~2010, I found it very easy to use a single author ID on each record, and have continued to solely use the first-listed author when entering books into the database. This works fine for cataloging and cross-referencing, but it can be sad sometimes because when looking up an author, you won't see any books they co-authored. The author field is actually much larger than necessary to store single author IDs because I expected to support this... and then never did.

It would be really nice to improve the software such that multiple author IDs can be added to a record and are handled appropriately by all pages.