issue 6265: add slovak translation for email and web templates

server/conf/index/standard_index.html_sk

server/conf/index/user_standard_index.html_sk

server/conf/mail/db_quota_notification_sk.txt

server/conf/mail/db_quota_ok_notification_sk.txt

server/conf/mail/mail_quota_notification_sk.txt

server/conf/mail/mail_quota_ok_notification_sk.txt

server/conf/mail/web_quota_notification_sk.txt

server/conf/mail/web_quota_ok_notification_sk.txt

server/conf/mail/web_traffic_notification_sk.txt

server/conf/mail/web_traffic_notification_warn_sk.txt

server/conf/mail/welcome_email_sk.txt