Add mail_plugins to separate dovecot config file which is included earlier

Update: mail_plugins (and postmaster address?) are refered to in the service blocks from the ISPConfig template. A script should grab them from the conf-custom file, comment them out in the 99-ispconfig-custom-config.conf file and add them to a new file 98-ispconfig-custom-config.conf or something like that. This file should be included in the ISPConfig template before the service blocks.

I am now looking to the code of !1459 (merged) - This will break the implementation

Because it is included earlier, values after it will override the custom config, but the custom config should override the config in dovecot.conf ofcourse. I tested the order before the original MR.

$mail_plugins should be added, that's good!