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/defaults/main.yml

@@ -408,9 +408,6 @@ matrix_ssl_lets_encrypt_certbot_docker_image: "certbot/certbot:v0.29.1"
 matrix_ssl_lets_encrypt_certbot_standalone_http_port: 2402
 matrix_ssl_lets_encrypt_support_email: "{{ host_specific_matrix_ssl_lets_encrypt_support_email }}"
 
-# Specifies when to attempt to retrieve new SSL certificates from Let's Encrypt.
-matrix_ssl_lets_encrypt_renew_cron_time_definition: "15 4 */5 * *"
-
 matrix_ssl_base_path: "{{ matrix_base_data_path }}/ssl"
 matrix_ssl_config_dir_path: "{{ matrix_ssl_base_path }}/config"
 matrix_ssl_log_dir_path: "{{ matrix_ssl_base_path }}/log"