show link to clear plaintext email account passwords if enabled; set store-plaintext-passwords default to 'off'; fixes #557

Signed-off-by: Michael Kaufmann <d00p@froxlor.org>

install/froxlor.sql

@@ -542,7 +542,7 @@ opcache.interned_strings_buffer'),
 	('system', 'defaultip', '1'),
 	('system', 'phpappendopenbasedir', '/tmp/'),
 	('system', 'deactivateddocroot', ''),
-	('system', 'mailpwcleartext', '1'),
+	('system', 'mailpwcleartext', '0'),
 	('system', 'last_tasks_run', '000000'),
 	('system', 'nameservers', ''),
 	('system', 'mxservers', ''),