timezone: Reduce vertical padding on the label

This makes it match other pages.
language: Don't dim the distro logo

It just looks tacky.
assistant: Remove the size group on the navigation buttons

There's no reason for them to be the same size anymore, now they're on
different sides of the header bar.

This works around a weird bug where the Skip button is much bigger than
it has any right to be, which stretches out the Previous button.