Release 6.20.0: Added Slovak translations for standard mail folder names

[changerequest]

The configuration file /opt/open-xchange/etc/admindaemon/User.properties will be extended with the following lines:

# Slovakia

SENT_MAILFOLDER_SK_SK=Odoslan\u00e9 polo\u017eky
TRASH_MAILFOLDER_SK_SK=K\u00f4\u0161
DRAFTS_MAILFOLDER_SK_SK=Rozp\u00edsan\u00e9
SPAM_MAILFOLDER_SK_SK=Spam
CONFIRMED_SPAM_MAILFOLDER_SK_SK=Potvrden\u00fd spam
CONFIRMED_HAM_MAILFOLDER_SK_SK=Potvrden\u00e9 ako nie spam