dovecot: migrated config to 2.4 + config splitting

data/conf/dovecot/conf.d/12-mysql.conf

@@ -0,0 +1,8 @@
+# Autogenerated by mailcow - DO NOT TOUCH!
+mysql /var/run/mysqld/mysqld.sock {
+  dbname=mailcow
+  user=mailcow
+  password=D8O9BIivJc7Pb2VCfpAeLbAzUOZ0
+
+  ssl = no
+}