If the user still has the API token that's generated from secret.key,
don't accept that.

Hopefully this is the last fix

As an AdministrativeMonitor, it shows up in the manage Jenkins UI, and
allows the administrator to run a re-keying operation.

We are replacing it by the ConfidentialStore class and the
ConfidentialKey class, which provides purpose-specific confidential
information that are separated from each other.

In this way, not all eggs are in one basket, and in case of a
compromise, the impact will contained.

Also replaced several insecure use of digest(secret|messsage) or
digest(message|secret) by HMAC.

This reverts commit adb562aa.