Fixes #586 (Github Issue)

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.

This is mostly here to guard against problems happening
due to server migration and doing `chown -R matrix:matrix /matrix`.

Normally, the file is owned by `1000:1000`, as expected.

If ownership changes, Dimension could still start, but it will fail the
first time it tries to write to the database. Explicitly chowning
before startup guards against this.

Related to #485 and #486 (Github Pull Requests).
Also related to ccc7aaf0ce.

Dimension runs as the `node` user in the container (`1000:1000`).
It doesn't seem like we have a way around it. Thus, its configuration
must also be readable by that user (or group, in this case).

No migration steps should be required.

Ansible will migrate the ownership of the base path and config path, but
manual intervention will be required in order to migrate the ownership
of files in those directories (i.e. dimension.db).

Stop the services:

  (local)$ ansible-playbook -i inventory/hosts setup.yml --tags=stop

Fix the permissions on the server:

  (server)# chown -Rv "{{ matrix_user_username }}:{{ matrix_user_username }}" "{{ matrix_dimension_base_path }}"

which would typically look like:

  (server)# chown -Rv matrix:matrix /matrix/dimension/

Reconfigure Dimension and start the services:

  (local)$ ansible-playbook -i inventory/hosts setup.yml --tags=setup-dimension,start

This playbook now suports running dimension in both a federated and an
unfederated environment.

This config change follows:

  https://github.com/turt2live/matrix-dimension/blob/master/docs/unfederated.md

Discussed in #282 (Github Issue).

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).

Fixes #171 (Github Issue).

Reasoning is the same as for matrix-org/synapse#5023.

For us, the journal used to contain `docker` for all services, which
is not very helpful when looking at them all together (`journalctl -f`).

Use Jinja2 lstrip_blocks option in templates to ensure consistent
indentation in generated files.

Media is pulled from client side, so we specify external Matrix DNS name as mediaUrl

It's been reported that YAML parsing errors
would occur on certain Ansible/Python combinations for some reason.

It appears that a bare `{{ matrix_dimension_admins }}` would sometimes
yield things like `[u'@user:domain.com', ..]` (note the `u` string prefix).

To prevent such problems, we now explicitly serialize with `|to_json`.