add arsnova base url to properties which is used for cas logout

redirection

src/main/webapp/config.properties.example

+security.arsnova-url=http://localhost:8080/arsnova-war
 security.openid-target-url=http://localhost:8080/arsnova-war/doOpenIdLogin
 security.cas-check-url=http://localhost:8080/arsnova-war/j_spring_cas_security_check
 security.cas-server-url=https://cas.thm.de/cas