Fix Timestamp Parsing Error that renders Invalid Datetime

[jyyyeung]

Fixes issue #206

1. Updated dateToZulu filter to return "" whenever the string is invalid
2. added fallback timestamp parsing functions from luxon when the default method fromISO() raises an error
3. Update sitemap to also use dateToZulu filter

[oleeskild]

awesome, thank you very much for contributing!

[BG6LH]

Thanks @jyyyeung !

I noticed that there might be an issue here. It seems that Luxon doesn't have a dateTime.fromHTML() function. So when I tried using a date format like 2025-03-12, 11:30:00, it didn't parse correctly and showed an error in the console.

Just wanted to bring this to your attention.