What hash is used to store the password should ideally be an
implementation detail, so that every user of the GUIConfiguration
object automatically agrees on how to handle it. That is currently
distribututed over the confighandler.go and api_auth.go files, plus
tests.
Add the SetHasedPassword() / CompareHashedPassword() API to keep the
hashing method encapsulated. Add a separate test for it and adjust
other users and tests. Remove all deprecated imports of the bcrypt
package.
