Update KOBO_IMAGEHOST_URL to new CDN endpoint

Kobo has introduced a vanity URL to their Akamai CDN config - "https://https://cdn.kobo.com". As a result, requests sent to the old raw Akamai endpoint now through 404s. This is a simple PR that updates to the new URL.

This is visible in the config of a newly bought Kobo Sage (mine):
```
image_host=https://cdn.kobo.com/book-images/
image_url_quality_template=https://cdn.kobo.com/book-images/{ImageId}/{Width}/{Height}/{Quality}/{IsGreyscale}/image.jpg
image_url_template=https://cdn.kobo.com/book-images/{ImageId}/{Width}/{Height}/false/image.jpg
```

Example:

OLD:
https://kbimages1-a.akamaihd.net/1abfb307-457b-4597-b2ea-74beb2eb1478/149/223/false/image.jpg
(Throws 404)

New:
https://cdn.kobo.com/book-images/1abfb307-457b-4597-b2ea-74beb2eb1478/149/223/false/image.jpg
(Renders Properly)

fixes #2371 
ref #2731

cps/kobo.py

@@ -56,7 +56,7 @@ from .kobo_auth import requires_kobo_auth, get_auth_token
 
 KOBO_FORMATS = {"KEPUB": ["KEPUB"], "EPUB": ["EPUB3", "EPUB"]}
 KOBO_STOREAPI_URL = "https://storeapi.kobo.com"
-KOBO_IMAGEHOST_URL = "https://kbimages1-a.akamaihd.net"
+KOBO_IMAGEHOST_URL = "https://cdn.kobo.com/book-images/"
 
 SYNC_ITEM_LIMIT = 100