dovecot: Add shared mailbox for the mediation team

Please see [1] and [2] for a better understanding of how this works.

[1] https://doc.dovecot.org/configuration_manual/authentication/password_databases_passdb/
[2] https://doc.dovecot.org/configuration_manual/authentication/user_databases_userdb/

Fix #464

roles/dovecot/files/shared-mailboxes

+mediation-team:::::::

roles/dovecot/files/shared-mailboxes-acl

+denisse_mediation-team:::::/home/vmail/shared-mailboxes/mediation-team::user=denisse
+dvzrv_mediation-team:::::/home/vmail/shared-mailboxes/mediation-team::user=dvzrv
+morganamilo_mediation-team:::::/home/vmail/shared-mailboxes/mediation-team::user=morganamilo

roles/dovecot/tasks/main.yml

@@ -13,6 +13,14 @@
   notify:
     - Reload dovecot
 
+- name: Install shared-mailboxes{,-acl}
+  copy: src={{ item }} dest=/etc/dovecot/ owner=root group=root mode=0644
+  loop:
+    - shared-mailboxes
+    - shared-mailboxes-acl
+  notify:
+    - Reload dovecot
+
 - name: Add vmail group
   group: name=vmail gid=5000
 

roles/dovecot/templates/dovecot.conf.j2

@@ -33,6 +33,10 @@ namespace inbox {
 	}
 }
 
+passdb {
+	driver = passwd-file
+	args = /etc/dovecot/shared-mailboxes-acl
+}
 passdb {
 	driver = pam
 }
@@ -89,6 +93,18 @@ ssl_options = no_compression
 
 ssl_cipher_list = ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384
 
+userdb {
+	driver = passwd-file
+	args = /etc/dovecot/shared-mailboxes
+	# https://doc.dovecot.org/configuration_manual/config_file/config_variables/
+	override_fields = uid=vmail gid=vmail home=/home/vmail/shared-mailboxes/%n
+}
+userdb {
+	driver = passwd-file
+	args = username_format=%{original_username} /etc/dovecot/shared-mailboxes-acl
+	# https://doc.dovecot.org/configuration_manual/config_file/config_variables/
+	override_fields = uid=vmail gid=vmail mail=mdbox:~/.mdbox:INDEXPVT=~/indexes/%Ln
+}
 userdb {
 	driver = passwd
 	# https://doc.dovecot.org/configuration_manual/config_file/config_variables/