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.

We mainly switch the anonymous metrics viewing variable
to false, along with other wording changes.

The quotes around "host" for both `--pid` and `--net` were
causing trouble for me:

> docker: --pid: invalid PID mode.

and:

> docker: Error response from daemon: network "host" not found.

I've also changed the `-v` call to `--mount` for consistency with the
rest of the playbook.

Fix wrong docker image being pulled

Changed `matrix_mautrix_signal_docker_image_force_pull` to `matrix_mautrix_signal_daemon_docker_image_force_pull` when force pulling the daemon

Using the hardcoded IP did break while I was
messing with IPv6 stuff on the other branch

By running it in a more privileged container with access to the host network stack and such

Also includes the dashboards for Synapse and for Node Exporter.

Again has only been tested on debian amd64 so far, but the grafana docker image is available for arm64 and arm32. Nice.

Basic system stats, to show stuff the synapse metrics
can't show such as resource usage by bridges, etc

Seems to work fine as well.

This too has only been tested on debian amd64 so far

Initial attempt. Seems to work fine.

Only tested on debian amd64 so far

This will make data migration easier.

Fixes: a66a604e ("Selfbuild appservice-slack bridge")

fix typo from skype to slack

Update configuring-dns.md

Added note about cloudflare, related to #821

Etherpad docs: Padname length bug

Warn users of the known bug https://github.com/turt2live/matrix-dimension/issues/395

I felt that adding another variable was probably going to be the easiest way to do this. I may end up adding another variable to enable this feature, for consistency with some of the other things.

Upgrade Element (1.7.19 -> 1.7.20)