chg: move PasswordChecker object to Gigi class

chg: move PasswordChecker object to Gigi class

This provides one centralized place where the PasswordChecker used can
be selected or changed. (setPasswordChecker() is intended for use in the
tests – in normal operation, the PasswordChecker should be set up once
during initialization and then not changed.)

I’d like to do this via dependency injection, but neither User nor
Signup seem like the right places to do this. Perhaps this kind of logic
should be moved to some kind of service where this is more feasible, but
that’s not a refactoring I want to do right now.

Change-Id: Id23d8a492a21a934085d9b91689acd723b065843