fix: manifest URL comparison

This will fix the finding of manifest indexes at several places in the app. Problem was that setting the item URL at the route caused URL encoding, so it was different from what is potentially returned from the API.

Entrypoint: https://dev.gfl.sub.uni-goettingen.de/tido/gfl/Z_1823-08-07_z/manifest.json

fix #403 (closed)