Enable Unattended Upgrades for stable-security

Since we now have a permanent infrastructure, we should at least be doing automated security updates. The simplest way to achieve this is to use unattended-upgrades.

The thing is, we don't want updates to run during a conference. Running updates at night could also be complex as "night" changes a lot depending on what timezone the online conference is.

Since u-u runs when the apt systemd timer (/lib/systemd/system/apt-daily-upgrade.timer) does, we could modify it so that it does not run when videoteam-record.service does. We can probably do this by adding Conflicts= and Before= statements to both the service and the time (see this).

At the moment, videoteam-record.service is a system user service, so we'll need to make it a global service first.

I also don't know how we can make sure not to forget to restart apt-daily-upgrade.timer when we stop videoteam-record.service, as we're 100% going to forget to start it manually. Maybe another timer that checks if we're recording and starts apt-daily-upgrade.timer if we're not?