Mailu: Enable server-side mail filtering (sieve support)

[Thorsten1976]

Support sieve/milter support, so that an user can configure server-side filters to avoid redundant filter setups on different clients.

Supported clients (remote configuration via sieve protocol) are roundcube or Thunderbird (via sieve addon).

• http://www.linux-magazin.de/ausgaben/2007/06/ausgesiebt/ (German)
• Sieve: http://sieve.info
• RFC 3028, „Sieve: A Mail Filtering Language“: http://tools.ietf.org/html/rfc3028
• Managesieve Draft: http://www.ietf.org/internet-drafts/draft-martin-managesieve-07.txt
• RFC 2033, „Local Mail Transfer Protocol“: http://www.ietf.org/rfc/rfc2033.txt
• Dovecot: https://wiki.dovecot.org/Pigeonhole/Sieve
• Sieve examples by Marcel Holtmann: http://www.holtmann.org/email/sieve/ (German)
• Avelsieve: http://email.uoa.gr/projects/squirrelmail/avelsieve.php
• Websieve: http://sourceforge.net/projects/websieve
• Roundcube: http://www.smartdomotik.com/2015/09/04/how-to-set-filters-in-roundcube-with-sieve/
• Thunderbird Sieve Addon: https://github.com/thsmi/sieve
• Sieve-Support for Emacs: http://josefsson.org/sieve/
• Sieve-Support for Mulberry: http://www.bath.ac.uk/bucs/email/sieve/mulberry-sieve.shtml
• Sieve Extension Support Matrix: http://wiki.fastmail.fm/wiki/index.php/SieveExtensionsSupportMatrix

[kaiyou]

The main current issue with Sieve is Mailu fonrtend and poor long term support.
I definitely agree about having a server-side filtering system, unfortunately sive is not supported in nginx as a mail reverse proxy. Also, no server supports proxying sieve and having TLS. @mildred opened a PR #428 on the topic, but we still need to have a proper configuration wizard before we merge this.

Another approach I like is converging to a flexible filtering configuration interface in the admin UI.

[kaiyou]

I will start enabling this on top of #612

[kaiyou]

Sieve scripts should now be officially available from the webmail.

[benbucksch]

Thank you for this wonderful feature! :1st_place_medal: