Release 6.20.1: Added new property to mailfilter.properties to support punycode conversion

[changerequest]

To support IDN domains in the mailfilter implementation which are provisioned with umlauts the mailfilter.properties file got a new property which enables the user to convert these umlauts to punycode. The following property has been added:

-------------------------------------------------------------------
# Enable punycode encoding for the username used in authentication against the managesieve server
com.openexchange.mail.filter.punycode=false
-------------------------------------------------------------------