Replace cronjobs with systemd timers

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/756

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/737

I feel like timers are somewhat more complicated and dirty (compared to
cronjobs), but they come with these benefits:

- log output goes to journald
- on newer systemd distros, you can see when the timer fired, when it
will fire, etc.
- we don't need to rely on cron (reducing our dependencies to just
systemd + Docker)

Cronjobs work well, but it's one more dependency that needs to be
installed. We were even asking people to install it manually
(in `docs/prerequisites.md`), which could have gone unnoticed.

Once in a while someone says "my SSL certificates didn't renew"
and it's likely because they forgot to install a cron daemon.

Switching to systemd timers means that installation is simpler
and more unified.

roles/matrix-coturn/tasks/setup_uninstall.yml

@@ -1,11 +1,5 @@
 ---
 
-# A similar task exists in `setup_install.yml`
-- name: Ensure matrix-coturn-ssl-reload cronjob removed
-  file:
-    path: /etc/cron.d/matrix-coturn-ssl-reload
-    state: absent
-
 - name: Check existence of matrix-coturn service
   stat:
     path: "{{ matrix_systemd_path }}/matrix-coturn.service"
@@ -17,28 +11,37 @@
     name: matrix-coturn
     state: stopped
     daemon_reload: yes
-  register: stopping_result
-  when: "not matrix_coturn_enabled|bool and matrix_coturn_service_stat.stat.exists"
+  when: "matrix_coturn_service_stat.stat.exists|bool"
 
-- name: Ensure matrix-coturn.service doesn't exist
+- name: Ensure matrix-coturn-reload.timer is stopped
+  service:
+    name: matrix-coturn
+    state: stopped
+    daemon_reload: yes
+  failed_when: false
+  when: "matrix_coturn_service_stat.stat.exists|bool"
+
+- name: Ensure systemd units don't exist
   file:
-    path: "{{ matrix_systemd_path }}/matrix-coturn.service"
+    path: "{{ matrix_systemd_path }}/{{ item }}"
     state: absent
-  when: "not matrix_coturn_enabled|bool and matrix_coturn_service_stat.stat.exists"
+  register: matrix_coturn_systemd_unit_uninstallation_result
+  with_items:
+    - matrix-coturn.service
+    - matrix-coturn-reload.service
+    - matrix-coturn-reload.timer
 
-- name: Ensure systemd reloaded after matrix-coturn.service removal
+- name: Ensure systemd reloaded after unit removal
   service:
     daemon_reload: yes
-  when: "not matrix_coturn_enabled|bool and matrix_coturn_service_stat.stat.exists"
+  when: "matrix_coturn_systemd_unit_uninstallation_result.changed|bool"
 
 - name: Ensure Matrix coturn paths don't exist
   file:
     path: "{{ matrix_coturn_base_path }}"
     state: absent
-  when: "not matrix_coturn_enabled|bool"
 
 - name: Ensure coturn Docker image doesn't exist
   docker_image:
     name: "{{ matrix_coturn_docker_image }}"
     state: absent
-  when: "not matrix_coturn_enabled|bool"