Install Horde 5 Webmail for ISPConfig on Debian Jessie through PEAR

Horde is a groupware suite that offers email, calendar, task, contact management and more things. It also offers SyncML and ActiveSync to synchronize with your cell phones or other software. This Howto assumes that you followed the ISPConfig 3 installation for Debian Jessie (Apache2, Bind, Dovecot) and all the packages there.