Add Mumble server

As per my announcement to arch-devops[1] and staff, this adds a Mumble
server for Arch Linux.

The password for the special root user SuperAdmin is automatically
generated on first launch and printed to the logs. I went ahead and
added it to the vault. It should not usually be required to login as
SuperAdmin though as long as there are user admins around.

This uses certbot for local certificates.

[1] https://lists.archlinux.org/archives/list/arch-devops@lists.archlinux.org/thread/AHAOSTGFJTLQDSXLWFORDKGR6RDVHYEI/