cors-anywhere no longer works as an open proxy

[pdille]

As per a pinned thread in the 'cors-anywhere' repo, their service is no longer offered as an open proxy and thus should no longer be used here.

Currently htmlpreview will give the following error:

Access to fetch at 'https://cors-anywhere.herokuapp.com/https://raw.githubusercontent.com/...' from origin 'https://htmlpreview.github.io' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.