dovecot: migrated config to 2.4 + config splitting

data/conf/dovecot/conf.d/50-quota.conf

@@ -0,0 +1,25 @@
+# /etc/dovecot/conf.d/50-quota.conf
+# Quota configuration and notifications.
+quota "User quota" {
+  driver = count
+
+  warning warn-95 {
+    quota_storage_percentage = 95
+    execute quota-warning {
+      args = 95 %{user}
+    }
+  }
+
+  warning warn-80 {
+    quota_storage_percentage = 80
+    execute quota-warning {
+      args = 80 %{user}
+    }
+  }
+}
+
+quota_clone {
+  dict proxy {
+    name = mysql_quota
+  }
+}