Enabled Managesieve Plugins for Dovecot and Roundcube

> c) Can you test that this doesn't interfere with the default sieve script to put spam in the spam folder?

The user's sieve script collection created by roundcube is stored in $STORAGE_ROOT/mail/managesieve not in $STORAGE_ROOT/mail/mailboxes so roundcube can't overwrite the other .dovecot.sieve.

See  #102