Roundcube managesieve plugin and automatic folder creation

Addresses #95

These are the changes necessary to enable Roundcube's manage sieve plugin to mange server-side sieve filters. I carefully followed the Dovecot + Roundcube instructions here: ISPmail tutorial for Debian Wheezy (https://workaround.org/ispmail/wheezy/) and made only the relevant changes that differed to get this working.

Also, I enabled Dovecot's autocomplete plugin to auto create and subscribe basic mail folders for each account so they appear right away in mail clients (only tested using Apple's Mail.app on Mac/iOS). These are: Drafts, Sent, Junk, Trash and Archive.