Matrix Authentication Support for Jitsi

This extends the collection with support for seamless authentication at the Jitsi server using Matrix OpenID.

1. New role for installing the [Matrix User Verification Service](https://github.com/matrix-org/matrix-user-verification-service)
2. Changes to Jitsi role: Installing Jitsi Prosody Mods and configuring Jitsi Auth
3. Changes to Jitsi and nginx-proxy roles: Serving .well-known/element/jitsi from jitsi.DOMAIN
4. We updated the Jitsi documentation on authentication and added documentation for the user verification service.

roles/custom/matrix-jitsi/templates/web/custom-config.js.j2

@@ -11,6 +11,10 @@ config.p2p.stunServers = [
 ];
 {% endif %}
 
+{% if matrix_jitsi_disable_gravatar %}
+config.gravatar = {'disabled': true};
+{% endif %}
+
 {% if matrix_jitsi_etherpad_enabled %}
 config.etherpad_base = {{ (matrix_jitsi_etherpad_base + '/p/') |to_json }}
 {% endif %}