Until now, we were leaving services "enabled"
(symlinks in /etc/systemd/system/multi-user.target.wants/).

We clean these up now. Broken symlinks may still exist in older
installations that enabled/disabled services. We're not taking care
to fix these up. It's just a cosmetic defect anyway.

Some people run Coturn or Jitsi, etc., by themselves and disable it
in the playbook.

Because the playbook is trying to be nice and clean up after itself,
it was deleting these Docker images.

However, people wish to pull and use them separately and would rather
they don't get deleted.

We could make this configurable for the sake of this special case, but
it's simpler to just avoid deleting these images.
It's not like this "cleaning things up" thing works anyway.
As time goes on, the playbook gets updated with newer image tags
and we leave so many images behind. If one doesn't run
`docker system prune -a` manually once in a while, they'd get swamped
with images anyway. Whether we leave a few images behind due to the lack
of this cleanup now is pretty much irrelevant.

No migration steps should be required.

We do this for 2 reasons:

- so we can control things which are not controllable using environment
variables (for example `stunServers` in jitsi/web, since we don't wish
to use the hardcoded Google STUN servers if our own Coturn is enabled)

- so playbook variable changes will properly rebuild the configuration.
When using Jitsi environment variables, the configuration is only built
once (the first time) and never rebuilt again. This is not the
consistent with the rest of the playbook and with how Ansible operates.
We're not perfect at it (yet), because we still let the Jitsi containers
generate some files on their own, but we are closer and it should be
good enough for most things.

Related to #415 (Github Pull Request).