Get file object from preferences instead from signal for generating favicons after preference change

This was done to ease testing without having to generate and configure
an RSA key. OpenID Connect is fully functional even without an RSA
key, although operating without RSA is not recommended.

On the go, support for directly passing keys in the configuration,
e.g. when configuring AlekSIS from a K8s secret, was added.

https://github.com/jazzband/django-oauth-toolkit/issues/1066