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-prometheus/templates/prometheus.yml.j2

@@ -37,11 +37,12 @@ scrape_configs:
         index: 0
   {% for worker in matrix_prometheus_scraper_synapse_workers_enabled_list %}
   {% if worker.metrics_port != 0 %}
-    - targets: ['matrix-synapse-worker-{{ worker.type }}-{{ worker.instanceId }}:{{ worker.metrics_port }}']
+    - targets: ['{{ worker.name }}:{{ worker.metrics_port }}']
       labels:
         instance: {{ matrix_domain }}
+		worker_id: {{ worker.id }}
         job: {{ worker.type }}
-        index: {{ worker.instanceId }}
+		app: {{ worker.app }}
   {% endif %}
   {% endfor %}
   {% endif %}