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.

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.

The matrix-nginx-proxy role can now be used independently.
This makes it consistent with all other roles, with
the `matrix-base` role remaining as their only dependency.

Separating matrix-nginx-proxy was relatively straightforward, with
the exception of the Mautrix Telegram reverse-proxying configuration.
Mautrix Telegram, being an extension/bridge, does not feel important enough
to justify its own special handling in matrix-nginx-proxy.

Thus, we've introduced the concept of "additional configuration blocks"
(`matrix_nginx_proxy_proxy_matrix_additional_server_configuration_blocks`),
where any module can register its own custom nginx server blocks.

For such dynamic registration to work, the order of role execution
becomes important. To make it possible for each module participating
in dynamic registration to verify that the order of execution is
correct, we've also introduced a `matrix_nginx_proxy_role_executed`
variable.

It should be noted that this doesn't make the matrix-synapse role
dependent on matrix-nginx-proxy. It's optional runtime detection
and registration, and it only happens in the matrix-synapse role
when `matrix_mautrix_telegram_enabled: true`.