This is both for consistency with 93cc71cb69976c
and for making things more obvious.

We'd like the roles to be self-contained (as much as possible).

Thus, the `matrix-nginx-proxy` shouldn't reference any variables from
other roles. Instead, we rely on injection via
`group_vars/matrix_servers`.

Related to #681 (Github Pull Request)

This broke in 63a49bb2dc.

Proxying the OpenID Connect endpoints is now possible,
but needs to be enabled explicitly now.

Supersedes #702 (Github Pull Request).

This patch builds up on the idea from that Pull Request,
but does things in a cleaner way.

Fixes #696 (Github Issue)

We do this to match Synapse's new default "max_upload_size" (50MB).

This `matrix_nginx_proxy_proxy_matrix_client_api_client_max_body_size_mb`
default value only affects standalone usage of the `matrix-nginx-proxy`
role. When the role is used in the context of the playbook,
the value is dynamically assigned from `group_vars/matrix_servers`.

Somewhat related to #692 (Github Issue).

Fixes #685 (Github Issue).

Fixes #586 (Github Issue)

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

Discussed in #101 (Github Issue).

Fixes #330 (Github Issue).

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).

As discussed in #151 (Github Pull Request), it's
a good idea to not selectively apply casting, but to do it in all
cases involving arithmetic operations.

Use an int conversion in the computation of the value of
matrix_nginx_proxy_tmp_directory_size_mb, to have the integer value
multiplied by 50 instead of having the string repeated 50 times.