add backwards compatible upgrade of method to store hashed passwords

I pulled out the password/hash related stuff into a separe PasswordService class and added testcases. I also did a manual backwards compatibility test with pre-patch db locally (automating that test would require loosening the dependency to BonobGitServerContext)
Thus the patch should be ready to be integrated fixing the https://github.com/jakubgarfield/Bonobo-Git-Server/issues/111 Issue.