The migration is automatic. Existing users should experience a bit of
downtime until the playbook runs to completion, but don't need to do
anything manually.

This change is provoked by https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2535

While my statements there ("Traefik is a shared component among
sibling/related playbooks and should retain its global
non-matrix-prefixed name and path") do make sense, there's another point
of view as well.

With the addition of docker-socket-proxy support in bf2b54080789f7e,
we potentially introduced another non-`matrix-`-prefixed systemd service
and global path (`/devture-container-socket-proxy`). It would have
started to become messy.

Traefik always being called `devture-traefik.service` and using the `/devture-traefik` path
has the following downsides:

- different playbooks may write to the same place, unintentionally,
  before you disable the Traefik role in some of them.
  If each playbook manages its own installation, no such conflicts
  arise and you'll learn about the conflict when one of them starts its
  Traefik service and fails because the ports are already in use

- the data is scattered - backing up `/matrix` is no longer enough when
  some stuff lives in `/devture-traefik` or `/devture-container-socket-proxy` as well;
  similarly, deleting `/matrix` is no longer enough to clean up

For this reason, the Traefik instance managed by this playbook
will now be called `matrix-traefik` and live under `/matrix/traefik`.

This also makes it obvious to users running multiple playbooks, which
Traefik instance (powered by which playbook) is the active one.
Previously, you'd look at `devture-traefik.service` and wonder which
role was managing it.

This paves the way for installing other roles into `roles/galaxy` using `ansible-galaxy`,
similar to how it's done in:

- https://github.com/spantaleev/gitea-docker-ansible-deploy
- https://github.com/spantaleev/nextcloud-docker-ansible-deploy

In the near future, we'll be removing a lot of the shared role code from here
and using upstream roles for it. Some of the core `matrix-*` roles have
already been extracted out into other reusable roles:

- https://github.com/devture/com.devture.ansible.role.postgres
- https://github.com/devture/com.devture.ansible.role.systemd_docker_base
- https://github.com/devture/com.devture.ansible.role.timesync
- https://github.com/devture/com.devture.ansible.role.vars_preserver
- https://github.com/devture/com.devture.ansible.role.playbook_runtime_messages
- https://github.com/devture/com.devture.ansible.role.playbook_help

We just need to migrate to those.

We don't have instantiated services anymore, nor
/etc/systemd/system/matrix-synapse.service.wants/ stuff.

This removes some `multi-target.wants` symlinks as well, etc.

But despite systemd saying:

> Removed symlink /etc/systemd/system/matrix-synapse.service.wants/matrix-synapse-worker@appservice:0.service

.. I still see such symlinks tehre for me for some reason, so keeping the
code (below) to find & delete them still seems like a good idea.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/756

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/737

I feel like timers are somewhat more complicated and dirty (compared to
cronjobs), but they come with these benefits:

- log output goes to journald
- on newer systemd distros, you can see when the timer fired, when it
will fire, etc.
- we don't need to rely on cron (reducing our dependencies to just
systemd + Docker)

Cronjobs work well, but it's one more dependency that needs to be
installed. We were even asking people to install it manually
(in `docs/prerequisites.md`), which could have gone unnoticed.

Once in a while someone says "my SSL certificates didn't renew"
and it's likely because they forgot to install a cron daemon.

Switching to systemd timers means that installation is simpler
and more unified.

Also switches to using `docker system prune -a` for a less invasive
cleanup of Docker images and related resources.

Use Jinja2 lstrip_blocks option in templates to ensure consistent
indentation in generated files.

Use matrix_docker_network and matrix_base_data_path in matrix-remove-all
instead of hardcoded default values.

Fixes #83 (Github issue)

As suggested in #63 (Github issue), splitting the
playbook's logic into multiple roles will be beneficial for
maintainability.

This patch realizes this split. Still, some components
affect others, so the roles are not really independent of one
another. For example:
- disabling mxisd (`matrix_mxisd_enabled: false`), causes Synapse
and riot-web to reconfigure themselves with other (public)
Identity servers.

- enabling matrix-corporal (`matrix_corporal_enabled: true`) affects
how reverse-proxying (by `matrix-nginx-proxy`) is done, in order to
put matrix-corporal's gateway server in front of Synapse

We may be able to move away from such dependencies in the future,
at the expense of a more complicated manual configuration, but
it's probably not worth sacrificing the convenience we have now.

As part of this work, the way we do "start components" has been
redone now to use a loop, as suggested in #65 (Github issue).
This should make restarting faster and more reliable.