We'be already been going against upstream defaults and have been
enabling backfilling for a few other bridges (Messenger, Instagram, Telegram, Twitter).

Now I'm enabling backfilling by default for the remaining ones, for
consistency.

Since upgrading mautrix-slack (and pinning to v0.1.0) in e4b54c37fe258b17f49dd7ed58a18ef3abac1c41,
we expect double-puppeting to require the new appservice double-puppeting method.

This commit switches the mautrix-slack bridge to it.

Related to:
- https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/3479
- https://github.com/mautrix/slack/releases/tag/v0.1.0
- https://mau.fi/blog/2024-08-mautrix-release/

Related to 0241c71a4c

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/3270#issuecomment-2143782962

With this change, it should be possible for people to adjust the Docker
dependency from `docker.service` to something else (e.g. `pkg-ContainerManager-dockerd.service`),
or to completely eliminate it by setting `devture_systemd_docker_base_docker_service_name` to an empty string.

This makes it easier for people to use the playbook against a Synology DSM server.

Signed-off-by: gardar <gardar@users.noreply.github.com>

This is backward-compatible with what we had before. We're not changing
the SSL mode - just making it configurable.

Most components are defaulting to `sslmode=disable`, while some
(`matrix-bot-matrix-reminder-bot` and others) do not specify an `sslmode` at all.

We're making sslmode configurable, because certain external Postgres
servers may be configured to require SSL encryption.
In such cases `sslmode=disable` does not work and needs to be changed to
`sslmode=require` or something else (`verify-ca`, `verify-full`, etc).

* one var to control encryption across all bridges

* move var to matrix-base