[nl] Enhance Dutch translation

[christianberkman]

Description
Found a few translations are technically correct, however very formal. Changed to more day-to-day translations.

• Years --> Dutch rules say use singular noun after number, so always "jaar"
• Hours --> Dutch rules say use singular noun after number, so always "uur"
• Now --> used more common "Zojuist"

Also added missing Cookies.invalidCookieValue

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide