ssh: introduce 'kallithea-cli ssh-update-authorized-keys' command for updating authorized_keys file

Based on work by Ilya Beda <ir4y.ix@gmail.com> on
https://bitbucket.org/ir4y/rhodecode/commits/branch/ssh_server_support ,
incorporating gearbox support by Anton Schur <tonich.sh@gmail.com> and also
heavily modified by Mads Kiilerich.

This commit also incorporates a fix for Windows by Dominik Ruf,
and better handling of the case where the parent dir of 'authorized_keys'
does not exist or is not writable, by Bradley M. Kuhn <bkuhn@ebb.org>.