- forego removing Docker images - it's not effective anyway, because it
  only removes the last version.. which is a drop in the bucket, usually

- do not reload systemd - it's none of our business. `--tags=start`,
  etc., handle this

- combine all uninstall tasks under a single block, which only runs if
  we detect traces (a leftover systemd .service file) of the component.
  If no such .service is detected, we skip them all. This may lead to
  incorect cleanup in rare cases, but is good enough for the most part.

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.

Adds support for: https://src.miscworks.net/fair/matrix-appservice-kakaotalk

This is pretty similar to
https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1977
which just appeared, but has mostly been done independently.

I've taken some inspiration and did some fixups based on that PR.
Thanks to https://github.com/hnarjis for taking the time to contribute!

Notable differences between this branch compared to that PR:

- better naming and documentation around the "configuration" variables
- no unnecessary (5 sec.) intentional delay when starting `matrix-appservice-kakaotalk-node.service`
- stores configuration in `config/`, not in `data/`
- passes configuration as read-only and starts the bridge with (`--no-update`) to ensure no changes are made to it
- starts containers more securely - with `matrix:matrix` user:group (not `root`) and
  reduced capabilities (`--cap-drop=ALL`)
- uses `tcp` for communication between the "node" and the appservice (simpler than sharing unix sockets)
- `registration.yaml` which is closer to the one generated by `matrix-appservice-kakaotalk` (no `de.sorunome.msc2409.push_ephemeral` stuff, etc.)
- `registration.yaml` which is more customizable (customizable bot username and prefix for puppets - see `matrix_appservice_kakaotalk_appservice_bot_username` and `matrix_appservice_kakaotalk_user_prefix`)
- less fragile and more extensible bridge permissions configuration via `matrix_appservice_kakaotalk_bridge_permissions`. Doing `{% if matrix_admin %}` in the bridge configuration sometimes causes syntax problems (I hit some myself) and is not ideal. Other bridges should be redone as well.
- configurable command prefix for the bridge, instead of hardcoding `!kt` (see `matrix_appservice_kakaotalk_command_prefix`)
- logging that is more consistent with the rest of the playbook (console / journald only, no logging to files), as well as configurable log level (via `matrix_appservice_kakaotalk_logging_level`)
- somewhat more detailed documentation (`docs/configuring-playbook-bridge-appservice-kakaotalk.md`)
- removed some dead code (data relocation tasks from `tasks/setup_install.yml`, as well as likely unnecessary SQLite -> Postgres migration)