Fixed system notifications blocking the user panel
'S' (2nd/3rd/etc) will be dropped in jQuery UI's datepicker
WCF.Date.Time now properly handles time offsets

The calculation of relative times previously failed if both the client and the server use the timezone, but their actual time is slightly different. In that case both times could be different by a few seconds, causing times to be in the future, even though it was just a mismatch.

Fixes #1179
Added missing language variable
Merge branch 'master' of github.com:WoltLab/WCF