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.