Fixes #129 (Github Issue).

Unfortunately, we rely on `service_facts`, which is only available
in Ansible >= 2.5.

There's little reason to stick to an old version such as Ansible 2.4:
- some time has passed since we've raised version requirements - it's
time to move into the future (a little bit)
- we've recently (in 82b4640072) improved the way one can run
Ansible in a Docker container

From now on, Ansible >= 2.5 is required.

Inspired by #128 (Github Issue), we've created a new Docker image
to replace https://hub.docker.com/r/qmxme/ansible

Adding dnspython or dig to `qmxme/ansible` doesn't seem like a good
idea (that might be accepted by them), given that it's specific to our
use case. That's why we'll be maintaining our own image from now on.

When using Let's Encrypt SSL certificates, a cronjob is set up to
automatically renew them. Though it does require a `cron`-compatible
program on the server.

This fixes the error that is caused by the `/etc/cron.d` directory
not existing and the `ansible-cron` module trying to write out a
file there -- without checking if the directory exists first.

Related to #108 (Github Pull Request).

It didn't mention `matrix_appservice_discord_client_id` and
`matrix_appservice_discord_bot_token`, which makes it hard for
beginners.

Related to #105 (Github Pull Request).

Bridge is setup to work on the matrix side with this, but the discord invite link is not automatically generated.

`matrix_synapse_no_tls` is now implicit, so we've gotten rid of it.

The `homeserver.yaml.j2` template has been synchronized with the
configuration generated by Synapse v0.99.1 (some new options
are present, etc.)

We had something like that on the Server Delegation how-to page,
but it's better if we have it on the SSL certificates page.

Relocated there and improved linking.

Fixes #94 (Github Issue)

This supersedes #59 (Github Pull Request),
which was greatly beneficial in creating our sample Apache configuration.

This was mentioned in #27 (Github Pull Request),
but it's just now that the nginx Docker image actually supports
TLS v1.3 and we can enable it.

This reverts commit 0dac5ea508.

Relying on pyOpenSSL is the Ansible way of doing things, but is
impractical and annoying for users.

`openssl` is easily available on most servers, even by default.
We'd better use that.