Add config, filter and service packages and minor improvements

src/main/java/de/rtuni/ms/as/Application.java

src/main/java/de/rtuni/ms/as/config/JWTConfiguration.java

...ain/java/de/rtuni/ms/as/config/SecurityConfiguration.java

...as/filter/JWTUsernameAndPasswordAuthenticationFilter.java

...n/java/de/rtuni/ms/as/service/UserDetailsServiceImpl.java

src/main/resources/application.yml