Added guide to use Kolab with reverse proxy

Summary:
I added an administrator guide for using Kolab with a reverse SSL proxy.

I also changed some other pages to include information about configuration options that was hard to find before (and important for setups with a reverse proxy).

Question: the configuration contains various references to a kolab.conf(5) man page. Does/did this man page exist?

Reviewers: Documentation, sicherha

Reviewed By: sicherha

Subscribers: sicherha

Differential Revision: https://git.kolab.org/D1966