feat(webui): prefill author selection when bulk editing books

[noaione]

The current implementation is that if we select multiple books like this:

• One has some author added
• Another is empty, or some of them filled

It will keep the whole thing empty.

This PR should make the selection prefilled with what has been added.

https://user-images.githubusercontent.com/34302902/213866289-3260f26c-7f58-44f0-8d1c-6db980f6dc89.mp4

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Some problems need addressing: As an example, two books have the following filled in:

• Book 1: Writer, Penciller
• Book 2: (Same) Writer

~~Currently, the form will have Writer and Penciller prefilled. I don't know if you like this behaviour or prefer to leave the Penciller empty.~~

Changed to basically get the intersection of many books for each roles, so only authors that exist on multiple books will be prefilled on a roles.