core: Include timezone for CTCP TIME replies

Explicitly specify the default Qt::TextDate date/time format to allow
modifying it.

On Qt 5, include the timezone data.  This doesn't reduce privacy as
timezone data could be figured out before by manually comparing with
UTC, this just makes it easier for those who want that information
shared.

See https://doc.qt.io/qt-5/qdatetime.html#toString
And https://doc.qt.io/qt-5/qt.html#DateFormat-enum