Forward Overdrive requests to Kobo Store

When loading books from OverDrive, Kobo readers make requests to `/v1/products/books/external/<overdrive ids>`.
This adds a route to forward these requests to the Kobo Store, so that OverDrive still works when using Calibre-Web's Kobo sync server.

cps/kobo.py

@@ -860,6 +860,7 @@ def HandleUserRequest(dummy=None):
 @kobo.route("/v1/products/<dummy>/recommendations", methods=["GET", "POST"])
 @kobo.route("/v1/products/<dummy>/nextread", methods=["GET", "POST"])
 @kobo.route("/v1/products/<dummy>/reviews", methods=["GET", "POST"])
+@kobo.route("/v1/products/books/external/<dummy>", methods=["GET", "POST"])
 @kobo.route("/v1/products/books/series/<dummy>", methods=["GET", "POST"])
 @kobo.route("/v1/products/books/<dummy>", methods=["GET", "POST"])
 @kobo.route("/v1/products/dailydeal", methods=["GET", "POST"])