Make Spring Session Repository Configurable

[ghillert]

Currently we provide as a default a MapSessionRepository. However, we should also provide an option to support SQL-based session storage:

https://docs.spring.io/spring-session/docs/current/reference/html5/#httpsession-jdbc

Possibly, we should also document on how to enable Redis-based session storage (without having a hard Redis-dependency in the project). Not sure if that would be possible, though.