This allows us to eliminate the companion and decrease overhead for
simple servers which do not use workers.

This also updates validation tasks and documentation, pointing to
variables in the matrix-synapse role which don't currently exist yet
(e.g. `matrix_synapse_container_labels_client_synapse_admin_api_enabled`).

These variables will be added soon, as Traefik labels are added to the
`matrix-synapse` role. At that point, the `matrix-synapse-reverse-proxy-companion` role
will be updated to also use them.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2090