Schleuder should provide the possibility to replace the list-key, either by a newly generated one, or by a provided one.

The new key should optionally (true by default) be signed by the old key to make the transition easier for users.

The option should be provided by the API and be used by Webschleuder and SchleuderConf.

(This feature would also serve the feature request to be able to nuke list-keys in case of compromisation (which was never filed, only told).)