Allow storing a pre-hashed admin password

When duplicating a couch, it is difficult to copy the _config/admins/*
values. Storing the encoded value does not work because that value is
re-hashed when stored. (Your password is the literal string
"-pbkdf2-abcdef...".)

This change will store any config setting unmodified if ?raw=true is
in the query string.

Updating _config/admins/* already requires admin privileges, so there is
no change to the security.