Close #2258: sieve scripts should be utf8 encoded

towncrier/newsfragments/2650.bugfix

@@ -0,0 +1 @@
+Tell roundcube to use UTF8 instead of 'UTF7-IMAP' when creating sieve scripts.

webmails/roundcube/config/config.inc.php

@@ -25,6 +25,7 @@ $config['smtp_pass'] = '%p';
 
 // Sieve script management
 $config['managesieve_host'] = '{{ FRONT_ADDRESS or "front" }}:14190';
+$config['managesieve_mbox_encoding'] = 'UTF8';
 
 // roundcube customization
 $config['product_name'] = 'Mailu Webmail';