All services injected

Dependency injection implemented everywhere. This means that every
servlet has a new Service, they are not shared anymore. TODO: inject
Hibernate session to DatabaseService. The complexity of code is now
insane. Still no idea if this is the right way to inject dependencies.

User finding moved from CodeItemDeserializer to CodeServiceImpl. All
logic should be moved from servlets to one of the services.

Warning: if USE_DATABASE is set to false, DatabaseService will be null.
Always use USE_DATABASE check in service methods to avoid an exception.

ServerController is now an empty shell. Only USE_DATABASE remains.