Grimmory OPDS feed - cover images

[adman234]

I'm not sure if this is a bug or just grimmory possibly having a different OPDS API formatting, but it would be nice if book covers would show up in the OPDS browser.

https://grimmory.org/api#tag/opds

Per the API docs, the formatting should be as below for the book cover:

/api/v1/opds/{bookId}/cover