add: DelegatingPasswordChecker implementation

This PasswordChecker implementation delegates to several other checkers,
which lets us use a series of checkers (e. g. one which rates the
password’s strength and one that checks against a list of known weak
passwords) in place of one.

In theory, this would also let us split up the existing
PasswordStrengthChecker into two checkers, one grading the password
strength in general and one checking whether the password contains parts
of the name or the email address. However, this would remove the current
behavior where a password that contains part of the name or email can be
“redeemed” by being otherwise strong enough: DelegatingPasswordChecker
does not support any such kind of interoperation of checkers.

Change-Id: I1066ab11cac8c756a2972128257a65d29cd2d365