This adds into the Riot config.json the field
  'default_server_config.m.homeserver.server_name'
with, by default, the value of the playbook's 'matrix_domain' variable.

Riot displays this string in its login page and will now say 'Sign in to
your Matrix account on example.org' (the server name) instead of 'Sign
in ... on matrix.example.org' (the server domain-name).

This string can be configured by setting the playbook variable
  'matrix_riot_web_default_server_name'
to any string, so we can make Riot say for example 'Sign in ... on Our
Server'.

This change makes no difference in itself, but is a pre-requisite for
adding the 'server name' config, and the old syntax is deprecated.

Switching to the official image (vectorim/riot-web) should ensure:
- there's less breakage, as it's maintained by the same team as riot-web
- there's fewer actors we need to trust
- we can upgrade riot-web faster, as newer versions should be released
on Docker hub at the same time riot-web releases are made

Fixes #71 (Github Issue).

I've not found this welcome bot to work at all in my previous attempts.
It would simply not reply, even though federation works.

It seems like this is also a potential privacy issue, as per
https://gist.github.com/maxidorius/5736fd09c9194b7a6dc03b6b8d7220d0

We do use some `:latest` images by default for the following services:
- matrix-dimension
- Goofys (in the matrix-synapse role)
- matrix-bridge-appservice-irc
- matrix-bridge-appservice-discord
- matrix-bridge-mautrix-facebook
- matrix-bridge-mautrix-whatsapp

It's terribly unfortunate that those software projects don't release
anything other than `:latest`, but that's how it is for now.

Updating that software requires that users manually do `docker pull`
on the server. The playbook didn't force-repull images that it already
had.

With this patch, it starts doing so. Any image tagged `:latest` will be
force re-pulled by the playbook every time it's executed.

It should be noted that even though we ask the `docker_image` module to
force-pull, it only reports "changed" when it actually pulls something
new. This is nice, because it lets people know exactly when something
gets updated, as opposed to giving the indication that it's always
updating the images (even though it isn't).

Fixes #171 (Github Issue).

Reasoning is the same as for matrix-org/synapse#5023.

For us, the journal used to contain `docker` for all services, which
is not very helpful when looking at them all together (`journalctl -f`).

Use Jinja2 lstrip_blocks option in templates to ensure consistent
indentation in generated files.

This prevents Ansible from sometimes failing to decrypt vault variables