CORS issue in some IIIF manifests

Some manuscripts (e.g., A-KN 1010) seem to be imported correctly into Cantus Ultimus.

However, the requests done at the viewer trigger CORS-related errors:

```
Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://manuscripta.at/AT1000-15/AT5000-1010/AT5000-1010_VDS.jpg/512,0,488,512/244,256/0/default.jpg.
```

As far as I remember, we have a "IIIF manifest proxy" in the django project of Cantus Ultimus. This is what handles every request to external libraries, precisely to avoid any CORS-related issues. It seems to be working for other libraries, I don't know why it wouldn't for some of them.

Not sure exactly how many of these manuscripts trigger CORS errors, my guess is that all the ones coming from manuscripta.at.

I fetched the images from their site myself, everything seems to be served okay by the library. The problem is in our end.