This is provoked by de91fe933d94afcdbfdb0c6cf75a1dfa43dfdf43,
where I've added a few new labels and made it possible for people to
disable them.

In this patch, I'm making it possible to disable any of the old Traefik
labels in a similar way.

Related to:

- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/3409
- https://github.com/t2bot/matrix-media-repo/releases/tag/v1.3.5
- https://github.com/matrix-org/matrix-spec-proposals/pull/3916

Support for authenticated media routes is enabled by default, but
variables are in place to disable it if necessary.

This change has not been tested.

This moves the comments from being just in Jinja,
to actually ending up in the generated `labels` file,
which makes inspection of the final result easier.

Also, some new lines were added here and there to make labels
more legible.

The generated file may still include weird new-lines due to
various `if` statements yielding content or not, but that's not so ugly
anymore - now that we have proper start/end sections that are visible in
the final `labels` file.