Closes #5284

Adds LibriVox and Project Gutenberg links to author pages if a corresponding remote_id is present.

Technical

In addition to the /config/author.yml configurations outlined in #5093, two new identifiers must be added:

-   label: Project Gutenberg
    name: project_gutenberg
    notes: ''
    url: https://www.gutenberg.org/ebooks/author/@@@
-   label: LibriVox
    name: librivox
    notes: ''
    url: https://librivox.org/author/@@@

Testing

1. Add the correct LibriVox and Project Gutenberg IDs to an author's identifiers.
2. Confirm that the links are present in the "Links (outside Open Library)" section of the author page.
3. Confirm that the links navigate to the correct Project Gutenberg and LibriVox pages.

Screenshot

Stakeholders

@RayBB @cdrini