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.

For an unknown reason prometheus ignored the given "numeric" index and replaced it by 1. This made it not work properly, plus multiple workers of same types were not differentiable. With a "string" index, it works as intended.