add dovecot cleanup timer to purge mdbox files

While mbox and maildir files get deleted by dovecot right away, that's
not the case for mdbox files. Since they contain multiple mails at once
in a proprietary format rewriting is expensive. That's why this step
is done in a separate step outside the dovecot process.

roles/dovecot/files/dovecot-cleanup.service

+[Unit]
+Description=Daily Dovecot cleanup
+
+[Service]
+Type=oneshot
+ExecStart=/usr/bin/doveadm purge -A

roles/dovecot/files/dovecot-cleanup.timer

+[Unit]
+Description=Daily Dovecot cleanup
+
+[Timer]
+OnCalendar=00:05:00
+
+[Install]
+WantedBy=timers.target

roles/dovecot/tasks/main.yml

@@ -42,3 +42,18 @@
   when: configure_firewall
   tags:
     - firewall
+
+- name: install systemd timers
+  copy: src={{ item }} dest=/etc/systemd/system/{{ item }} owner=root group=root mode=0644
+  with_items:
+    - dovecot-cleanup.timer
+    - dovecot-cleanup.service
+
+- name: activate systemd timers
+  systemd:
+    name: "{{ item }}"
+    state: started
+    enabled: yes
+    daemon_reload: yes
+  with_items:
+    - dovecot-cleanup.timer