Svelte 5 Migration 7: migrate from askama to rinja

[sebadob]

A migration from askama to rinja makes sense before doing updates on SSR and <template> rendering for the new UI setup. rinja is a fork of askama with lots of improvements and additional features, and the same core people from askama do work on rinja as well.

rinja is also used to render docs.rs and even though Rauthy does not make use of extensive templating features, the performance difference is pretty huge:
https://github.com/rinja-rs/template-benchmark/

#642