Synapse workers

Adding more presets in the future would be nice.

We're talking about a webserver running on the same machine, which
imports the configuration files generated by the `matrix-nginx-proxy`
in the `/matrix/nginx-proxy/conf.d` directory.

Users who run an nginx webserver on some other machine will need to do
something different.

fix wrong json parameter in webhook documentation

Related to: https://matrix.org/blog/2021/02/18/synapse-1-27-0-released#dropping-armv7-docker-images

Add Netlify instructions

fixes HTTP 400 Error: "{\"errcode\":\"M_MISSING_PARAM\",\"error\":\"Missing key 'avatar_url'\"}"

Remove 'sqlite3' from the Etherpad role

The official Etherpad Docker image has no support for sqlite3 databases.

This give us the possibility to run multiple instances of
workers that that don't expose a port.

Right now, we don't support that, but in the future we could
run multiple `federation_sender` or `pusher` workers, without
them fighting over naming (previously, they'd all be named
something like `matrix-synapse-worker-pusher-0`, because
they'd all define `port` as `0`).

Use the yaml callback plugin when running ansible-playbook

The default default Ansible screen output encodes and prints error outputs as a hard to read dense line of JSON.
This patch changes the ansible-playbook command behavior for this project to output yaml instead.

Update synapse-admin (0.6.1 -> 0.7.0)

Change the new app link to use classsic apps

Until [this issue](https://github.com/matrix-org/matrix-appservice-slack/issues/348) is fixed

Related to:
- https://github.com/matrix-org/synapse/pull/9407
- https://github.com/matrix-org/synapse/pull/7855

Fix etherpad binding

don't bind to any host port if nginx_proxy is used
only bind to localhost if it's not used

This leads to much easier management and potential safety
features (validation). In the future, we could try to avoid port
conflicts as well, but it didn't seem worth the effort to do it now.
Our port ranges seem large enough.

This can also pave the way for a "presets" feature
(similar to `matrix_nginx_proxy_ssl_presets`) which makes it even easier
for people to configure worker counts.

`::` leads to errors like:

> socket.gaierror: [Errno -9] Address family for hostname not supported

Add additional domains for Let's Encrypt certificates to be obtained

These validation tasks should only run when Prometheus is enabled.

Prometheus and Grafana on stats.<domain>

.. and other documentation improvements.

It seems like it doesn't cause any issues for any of these services.