Draft: Add new role for RT5

The Ansible part (i. e., not the config template for RT itself) has been completely rewritten. As this is a new package in Debian and the upgrade involves some manual steps, this seemed like a good time to clean up the dubious parts in the old role and do it better from the beginning.

That being said, some points are still not quite up-to-date yet: the webservices/caddy role should be included by FQCN (which ansible-lint rightly complains about), however none of our shared roles are proper Ansible Collections yet.