Display default timezone on System Status

When a plugin is behaving badly and calling `date_default_timezone_set()` and changing the site's timezone from UTC, it causes all sorts of issues.

This patch displays the default timezone that is set when the System Status screen is loaded. That's not a guaranteed indicator of the default timezone used everywhere on the site, but it will reveal when an offending plugin is calling `date_default_timezone_set()` on `init` or similar  site wide hook.