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/cron.d/matrix-nginx-proxy-periodic-restarter.j2

@@ -1,8 +0,0 @@
-MAILTO="{{ matrix_ssl_lets_encrypt_support_email }}"
-
-# This periodically reloads the matrix-nginx-proxy service
-# to ensure it's using the latest SSL certificate
-# in case it got renewed by the `matrix-ssl-certificate-renewal` cronjob
-# (which happens once every ~2-3 months).
-
-{{ matrix_nginx_proxy_reload_cron_time_definition }} root /usr/bin/systemctl reload matrix-nginx-proxy.service

roles/matrix-server/templates/cron.d/matrix-ssl-certificate-renewal.j2

@@ -1,11 +0,0 @@
-MAILTO="{{ matrix_ssl_lets_encrypt_support_email }}"
-
-# The goal of this cronjob is to ask certbot to check
-# the current SSL certificates and to see if some need renewal.
-# If so, it would attempt to renew.
-#
-# Various services depend on these certificates and would need to be restarted.
-# This is not our concern here. We simply make sure the certificates are up to date.
-# Restarting of services happens on its own different schedule (other cronjobs).
-
-{{ matrix_ssl_lets_encrypt_renew_cron_time_definition }} root /bin/bash /usr/local/bin/matrix-ssl-certificates-renew