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.

This supersedes/fixes-up this Pull Request:
https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/719

The Jitsi Web and JVB containers now (in build 5142) always
start by bulding their own default configuration
(`config.js` and `sip-communicator.properties`, respectively).

The fact that we were generating these files ourselves was no longer of use,
because our configuration was thrown away in favor of the one created
by the containers on startup.

With this commit, we're completely redoing things. We no longer
generate these configuration files. We try to pass the proper
environment variables, so that Jitsi services can generate the
configuration files themselves.

Besides that, we try to use the "custom configuration" mechanism
provided by Jitsi Web and Jitsi JVB (`custom-config.js` and
`custom-sip-communicator.properties`, respectively), so that
we and our users can inject additional configuration.

Some configuration options we had are gone now. Others are no longer
controllable via variables and need to be injected using
the `_config_extension` variables that we provide.

The validation logic that is part of the role should take care
to inform people about how to upgrade (if they're using some custom
configuration, which needs special care now). Most users should not
have to do anything special though.

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).