the changes are necessary because certbot images are tagged, so docker does not recognize the necessary architecture

Well, actually 8cd9cde won't work, unless we put the
`|to_nice_yaml` thing on a new line.

We can, but that takes more lines and makes things look uglier.
Using `|to_json` seems good enough.

The whole file is parsed as YAML later on and merged with the
`_extension` variable before being dumped as YAML again in the end.

Should help with #300 (Github Issue).

Related to #451 (Github Issue).

This will not work yet, as no version of Riot currently supports it.
It's expected to land in riot-web v1.5.16 via matrix-org/matrix-react-sdk#4348.

Hopefully fixes an error like this (which I haven't been able to
reproduce, but..):

> [modules/xmpp/strophe.util.js] <Object.i.Strophe.log>:  Strophe: Error: Failed to construct 'RTCPeerConnection': 'matrix.DOMAIN' is not one of the supported URL schemes 'stun', 'turn' or 'turns'.

These are some new defaults that we switch to, to follow upstream
changes:

- https://github.com/jitsi/docker-jitsi-meet/commit/264df04
- 3121494d4b

We define this password in the `sip-communicator.properties`
configuration file, so this is not needed for actually running JVB.

However, it does a (useless) safety check during container startup,
and we need to make that check happy.

When the playbook executes on a remote host (as it usually does),
we need to explicitly say `src` is a remote file and not relative
to the playbook.

(adapting strings and URLs)

Discussed in #101 (Github Issue).

Fixes #427 (Github Issue).