This allows people to try out the new Element X clients, which need to
run against the sliding-sync proxy (https://github.com/matrix-org/sliding-sync).

Supersedes https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2515

The code is based on the existing PR (#2515), but heavily reworked. Major changes:

- lots of internal refactoring and variable renaming

- fixed self-building to support non-amd64 architectures

- changed to talk to the homeserver locally, over the container network (not
  publicly)

- no more matrix-nginx-proxy support due to complexity (see below)

- no more `matrix_server_fqn_sliding_sync_proxy` in favor of
  `matrix_sliding_sync_hostname` and `matrix_sliding_sync_path_prefix`

- runs on `matrix.DOMAIN/sliding-sync` by default, so it can tried
  easily without having to create new DNS records

The variable was necessary when multiple playbooks could have
potentially tried to manage a shared `devture-traefik.serivce` systemd service
and shared `/devture-traefik` directory.

Since adcc6d9723086f65f1a72, we use our own `/matrix/traefik`
(`matrix-traefik.service`) installation and no conflicts can arise.
It's safe to always enable the role, just like we do with all the other roles.

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 is a mistake made in 10b5350370.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2540

* healthchecks.io integration
* mutex on forwarding messages into thread
* fix in prefixes handling
* send error messages as thread reply when possible

Completely untested.

This new role also adds native Traefik support and support for other
(non-`amd64`) architectures via self-building.

Config was accidently changed when importing upstream changes.

We don't need these 2 roughly-the-same settings related to the
traefik-certs-dumper role.

For Traefik, it makes sense, because it's a component used by the
various related playbooks and they could step onto each other's toes
if the role is enabled, but Traefik is disabled (in that case, uninstall
tasks will run).

As for Traefik certs dumper, the other related playbooks don't have it,
so there's no conflict. Even if they used it, each one would use its own
instance (different `devture_traefik_certs_dumper_identifier`), so there
wouldn't be a conflict and uninstall tasks can run without any danger.

This helps people who run federation on another domain (etc.).
They should now easily be able to add a custom rule for federation traffic.

Fixes a regression introduced in 0220c851e8

BuildX is required for building this container image.

This makes it consistent with the rest of the playbook:
- there's a default config which has various variables controlling
  settings
- there's also an `_extension_yaml` variable, which lets you override it

We disable it via `group_vars/matrix_servers`, as we do for all other
roles.

The next task asks for a daemon reload, so there's no need to do it
twice.

Larger deployments may wish to run migration more often.