Add support for inviting telephone participants to meeting

The Jitsi-Meet invite to conference by phone feature is not showing in UI beacuse [pade does not support the required external web services to make it happen](https://community.jitsi.org/t/jitsi-meet-and-jigasi-outbound-number-format/14045). As a replacement, add a "invite by phone" button to the Contacts dialog when the SIP gateway is enabled.