matrix-synapse: Allow configuring preferred Jitsi server

Audio/video calls in Element for groups with greater than 2 people are handled using a Jitsi server. Element sets the default server to jitsi.riot.im

It is possible to set the default Jitsi server to be used for all users of a Matrix Synapse server as mentioned here. https://github.com/vector-im/element-web/blob/develop/docs/jitsi.md

This configuration is known to work in the Web and Android versions of Element.

A FreedomBox administrator should be able to set their preferred Jitsi server for use with audio/video conferences in Element.