Set up a Mumble server for the fundraising team

Originally created by @intrigeri on #12447 (Redmine)

Following up on tails/tails#11003: tails/tails#5688 is now planned for 2017Q4 and DYI self-hosted Mumble servers have shown their limits recently (e.g. due to poor or costly Internet connections, or due to too few people having bothered checking if they like the blueprint/mumble-server script enough to run it).

So as said on https://gitlab.tails.boum.org/tails/tails/-/issues/11003#note_64173 I might start with setting up one such server on im.lizard for one of our teams, and will skip the more complex discussion.

Related issues

• Related to tails/tails#11003
• Related to tails/tails#9851