[applets/digital clock] Restore scroll-to-change-displayed-timezone
After the rest of the UI changes made to the applet and the time zone config page, I don't think users will be confused by the differences between the timezone displayed in the clock, and the systemwide local timezone anymore. Therefore, it should be safe enough to bring back the wheely time feature.
The rest of the layout is very slightly adjusted to make room for the new checkbox:
cc @bshah @teams/vdg
Since this is targeting 5.21, it will require a feature+string freeze exception if approved.