If a service is enabled, a database for it is created in postgres with a uniqque password. The service can then use this database for data storage instead of relying on sqlite.

The Docker 19.04 -> 20.10 upgrade contains the following change
in `/usr/lib/systemd/system/docker.service`:

```
-BindsTo=containerd.service
-After=network-online.target firewalld.service containerd.service
+After=network-online.target firewalld.service containerd.service multi-user.target
-Requires=docker.socket
+Requires=docker.socket containerd.service
Wants=network-online.target
```

The `multi-user.target` requirement in `After` seems to be in conflict
with our `WantedBy=multi-user.target` and `After=docker.service` /
`Requires=docker.service` definitions, causing the following error on
startup for all of our systemd services:

> Job matrix-synapse.service/start deleted to break ordering cycle starting with multi-user.target/start

A workaround which appears to work is to add `DefaultDependencies=no`
to all of our services.

This is both for consistency with 93cc71cb69976c
and for making things more obvious.

Depending on the distro, common commands like sleep and chown may either
be located in /bin or /usr/bin.

Systemd added path lookup to ExecStart in v239, allowing only the
command name to be put in unit files and not the full path as
historically required. At least Ubuntu 18.04 LTS is however still on
v237 so we should maintain portability for a while longer.

No migration steps should be required.

We've had reports of the `:latest` tag not working anymore,
leading to an error as described here:
https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/341

This reverts commit 61e93744ec.

Reverting because of:
- https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/341
- https://github.com/matrix-org/matrix-appservice-slack/issues/347

NeDB is deprecated, so we should transition away from it in the future.
Still, most of the functionality should work on this new version.