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 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.

* fix: only add element related entries to client well-known if element is enabled

* Fix matrix-base/defaults/main.yml syntax

---------

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

* Enable location sharing in Element

* Update roles/custom/matrix-client-element/tasks/validate_config.yml

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

* Update roles/custom/matrix-client-element/tasks/setup_install.yml

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

* Rename location sharing vars to be consistent with other vars

* Rename style.json to map_style.json

* Add m.tile_server section to /.well-known/matrix/client

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

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.