matrix-synapse: Allow user-defined TURN server for VoIP calls

Created by: JoKeyser

Once a TURN server is part of FreedomBox (see #685 (closed)), Edit: We should add the option to enter STUN/TURN servers for matrix-synapse, which enables NAT traversal of VoIP calls if direct peer-to-peer fails. See https://github.com/matrix-org/synapse/blob/master/docs/turn-howto.rst