supplementary column heading (Trac 10718)
Optimize HTML layout

- Move Release box to the right border
- Make Debian-Mirror list span to the right border
Reinstate previous mirror list when "auto" is toggled/untoggled

Previously, the Debian mirror list got reset to empty after "auto" was
untoggled.  Instead, save the mirror list before setting "auto" and reinstate
the list again afterwards.
Merge remote-tracking branch 'lhm/2.7-lhm' into 2.7-lhm