allow adding additional private keys via schleuder-cli
we had one case, when a group of people wanted to write encrypted mails and decided to set up a ordinary mailinglist (lists.riseup.net) with one shared key.
our schleuder was member of this list and at first complaining heavily, because no key to decrypt was found. i managed to deal with this by adding this private key to the lists keyring manually using gpg with --homedir, --import and --edit-key (to set owner-trust - not sure if this was necessary).
in the end it worked, but i wasn't sure, if this would be a good idea and if schleuder would accept that.
it would be nice if this could be done in a more clean (and documented) way using schleuder-cli and/or schleuder-web.