[RCD] Improvements to the Managesieve plugin

[Aleksander Machniak]

1. Import of disabled rules generated by the SieveRules plugin
ManageSieve uses the trick "if false ..." to disable sieve rules, but
SieveRules stores disabled rules as a PHP serialized form of it's
internal representation in a sieve comment. Now we have a working
version for this that is able to import conditions and vacation
(autoreply) rules
2. Add original subject to vacation replies

Please, create two separate pull requests on
https://github.com/roundcube/roundcubemail/