Switch to managing cronjobs with the Ansible cron module

As suggested in #65 (Github issue), this patch switches
cronjob management from using templates to using Ansible's `cron` module.

It also moves the management of the nginx-reload cronjob to `setup_ssl_lets_encrypt.yml`,
which is a more fitting place for it (given that this cronjob is only required when
Let's Encrypt is used).

Pros:
- using a module is more Ansible-ish than templating our own files in
special directories

- more reliable: will fail early (during playbook execution) if `/usr/bin/crontab`
is not available, which is more of a guarantee that cron is working fine
(idea: we should probably install some cron package using the playbook)

Cons:
- invocation schedule is no longer configurable, unless we define individual
variables for everything or do something smart (splitting on ' ', etc.).
Likely not necessary, however.

- requires us to deprecate and clean-up after the old way of managing cronjobs,
because it's not compatible (using the same file as before means appending
additional jobs to it)

roles/matrix-server/templates/usr-local-bin/matrix-ssl-lets-encrypt-certificates-renew.j2

@@ -0,0 +1,26 @@
+#!/bin/bash
+
+# For renewal to work, matrix-nginx-proxy (or another webserver, if matrix-nginx-proxy is disabled)
+# need to forward requests for `/.well-known/acme-challenge` to the certbot container.
+#
+# This can happen inside the container network by proxying to `http://matrix-certbot:80`
+# or outside (on the host) by proxying to `http://localhost:{{ matrix_ssl_lets_encrypt_certbot_standalone_http_port }}`.
+
+docker run \
+	--rm \
+	--name=matrix-certbot \
+	--network="{{ matrix_docker_network }}" \
+	-p 127.0.0.1:{{ matrix_ssl_lets_encrypt_certbot_standalone_http_port }}:80 \
+	-v {{ matrix_ssl_config_dir_path }}:/etc/letsencrypt \
+	-v {{ matrix_ssl_log_dir_path }}:/var/log/letsencrypt \
+	{{ matrix_ssl_lets_encrypt_certbot_docker_image }} \
+	renew \
+		--non-interactive \
+		{% if matrix_ssl_lets_encrypt_staging %}
+			--staging \
+		{% endif %}
+		--quiet \
+		--standalone \
+		--preferred-challenges http \
+		--agree-tos \
+		--email={{ matrix_ssl_lets_encrypt_support_email }}