Add json format to SessMess.scala

When a route responds with a SessMess an implicit json format has to be
defined. By moving this format into an object SessMessJsonProtocol, it
only has to be defined once and imported where relevant.
Factor out services from Service.scala

Service.scala contains to many services that could be factored out to
more files. Add EmailService, GroupService, PasswordService and
SessionService and move the relevant services from Service.scala to the
appropriate files.