Add support for SOGo integrator frontend Thunderbird addon

This seems to be currently missing from version 0.9.0. Documentation: http://www.sogo.nu/files/docs/SOGo%20Mozilla%20Thunderbird%20Configuration.pdf

SOGo integrator would help in rolling our Thunderbird integration with add-ons, calendars, and address books driven by iredmail. This currently does not seem to work with the default iredmail configuration.

There's nothing iRedMail should do except integrating SOGo properly. You just need to download Thunderbird + add-ons and configure them properly.