Add TURN/STUN server

A TURN or STUN server running on FreedomBox can be useful for SIP clients that are behind a firewall.

There are a number of Debian packages providing TURN or STUN servers:

• coturn
• ejabberd (also supports SIP)
• resiprocate-turn-server
• rfc5766-turn-server
• stuntman-server

We will need to investigate which of these are suitable for use on FreedomBox, especially considering ease of configuration.