Add ISBNDB Support

[flamedfury]

Is your feature request related to a problem? Please describe.
I've just come across https://isbndb.com/ and thought it could be a great alternative to the Google Books provider.

Describe the solution you'd like
Implement https://isbndb.com/ to be available to be used in place of Google Books for physical and ebooks.

[katydecorah]

Hey! We did have support for this, but I ended up removing it because API access requires a subscription: https://isbndb.com/user/register I didn't have a way to test it since I don't use the service.

Are you a subscriber? If so, I can add this endpoint back! It just might be harder for me to test for regressions since the API isn't open.