Add a self-build option for the Dimension Dockerfile.
 - This helps further support arm64 hosts (i.e. A1.Flex w/Oracle)

Until now, we were leaving services "enabled"
(symlinks in /etc/systemd/system/multi-user.target.wants/).

We clean these up now. Broken symlinks may still exist in older
installations that enabled/disabled services. We're not taking care
to fix these up. It's just a cosmetic defect anyway.

We'll be adding `.timer` units later on, so it's good to be
more explicit.

This is 8b6174786b done right. There were many more fields
that we had to account for.

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.

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).