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.

Signed-off-by: Sebastian Gumprich <github@gumpri.ch>

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1939

This commit adds a 'matrix-ntfy' role that runs Ntfy server in Docker with
simple configuration, and plumbing to add the role to the playbook.

TODO: documentation, self-check, database persistence.

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.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/683

We do this by creating one more layer of indirection.

First we reach some generic vhost handling matrix.DOMAIN.
A bunch of override rules are added there (capturing traffic to send to
ma1sd, etc). nginx-status and similar generic things also live there.

We then proxy to the homeserver on some other vhost (only Synapse being
available right now, but repointing this to Dendrite or other will be
possible in the future).
Then that homeserver-specific vhost does its thing to proxy to the
homeserver. It may or may not use workers, etc.

Without matrix-corporal, the flow is now:
1. matrix.DOMAIN (matrix-nginx-proxy/matrix-domain.conf)
2. matrix-nginx-proxy/matrix-synapse.conf
3. matrix-synapse

With matrix-corporal enabled, it becomes:
1. matrix.DOMAIN (matrix-nginx-proxy/matrix-domain.conf)
2. matrix-corporal
3. matrix-nginx-proxy/matrix-synapse.conf
4. matrix-synapse

(matrix-corporal gets injected at step 2).

Fixes #586 (Github Issue)

No migration steps should be required.

We do use some `:latest` images by default for the following services:
- matrix-dimension
- Goofys (in the matrix-synapse role)
- matrix-bridge-appservice-irc
- matrix-bridge-appservice-discord
- matrix-bridge-mautrix-facebook
- matrix-bridge-mautrix-whatsapp

It's terribly unfortunate that those software projects don't release
anything other than `:latest`, but that's how it is for now.

Updating that software requires that users manually do `docker pull`
on the server. The playbook didn't force-repull images that it already
had.

With this patch, it starts doing so. Any image tagged `:latest` will be
force re-pulled by the playbook every time it's executed.

It should be noted that even though we ask the `docker_image` module to
force-pull, it only reports "changed" when it actually pulls something
new. This is nice, because it lets people know exactly when something
gets updated, as opposed to giving the indication that it's always
updating the images (even though it isn't).

In most cases, there's not really a need to touch the system
firewall, as Docker manages iptables by itself
(see https://docs.docker.com/network/iptables/).

All ports exposed by Docker containers are automatically whitelisted
in iptables and wired to the correct container.

This made installing firewalld and whitelisting ports pointless,
as far as this playbook's services are concerned.

People that wish to install firewalld (for other reasons), can do so
manually from now on.

This is inspired by and fixes #97 (Github Issue).

`matrix_nginx_proxy_data_path` has always served as a base path,
so we're renaming it to reflect that.

Along with this, we're also introducing a new "data path" variable
(`matrix_nginx_proxy_data_path`), which is really a data path this time.
It's used for storing additional, non-configuration, files related to
matrix-nginx-proxy.

Fixes #69 (Github Issue)