Add support for stream writer Synapse workers

As stream writer workers are also powered by the `generic_worker`
Synapse app, this necessitated that we provide means for distinguishing
between them and regular `generic_workers`.

I've also taken the time to optimize nginx configuration generation
(more Jinja2 macro usage, less duplication).

Worker names have also changed.
Workers are now named sequentially like this:
- `matrix-synapse-worker-0-generic`
- `matrix-synapse-worker-1-stream-writer-typing`
- `matrix-synapse-worker-2-pusher`

instead of `matrix-synapse-worker_generic_worker-18111` (indexed with a
port number).

People who modify `matrix_synapse_workers_enabled_list` directly will
need to adjust their configuration.

roles/matrix-synapse/templates/synapse/systemd/matrix-synapse-worker.service.j2

@@ -43,7 +43,7 @@ ExecStart={{ matrix_host_command_docker }} run --rm --name {{ matrix_synapse_wor
 			{{ arg }} \
 			{% endfor %}
 			{{ matrix_synapse_docker_image }} \
-			run -m synapse.app.{{ matrix_synapse_worker_details.type }} -c /data/homeserver.yaml -c /data/{{ matrix_synapse_worker_config_file_name }}
+			run -m synapse.app.{{ matrix_synapse_worker_details.app }} -c /data/homeserver.yaml -c /data/{{ matrix_synapse_worker_config_file_name }}
 
 
 ExecStop=-{{ matrix_host_command_sh }} -c '{{ matrix_host_command_docker }} kill {{ matrix_synapse_worker_container_name }} 2>/dev/null || true'