www/baikal: import IMAP auth backend from sabre/dav master

To authenticate against an IMAP server, you'll need to:

1. Change the authBackend in Core/Frameworks/Baikal/Core/Server.php:135 to $authBackend = new \Sabre\DAV\Auth\Backend\IMAP(IMAP_OPEN);

2. Set the IMAP_OPEN configuration entry in Specific/config.php, e.g., define("IMAP_OPEN", '{192.168.1.1:993/imap/ssl/novalidate-cert}INBOX');

Obtained from: https://github.com/fruux/sabre-dav/blob/master/lib/DAV/Auth/Backend/IMAP.php