Add support for shibboleth login

[Luro02]

Login has become a lot more complicated and error prone with the switch and now multiple ways to login.

In the future one might forbid login through direct credentials or prevent the creation of new ones,
so it makes sense to support login through shibboleth.

There exists a library called shibboleth-authenticator, which might work, but seems to be unmaintained.
This project has python code for shibboleth login as well and is maintained:

https://github.com/Garmelon/PFERD/blob/6d44aac2783c69031e7686263fc0a2285912376f/PFERD/crawl/ilias/kit_ilias_web_crawler.py#L818-L982