Bug 35097: Use country-list.inc to display choices for UsageStatsCountry preference

Bug 35097: Use country-list.inc to display choices for UsageStatsCountry preference

This patch updates the system preferences interface so that it uses
country-list.inc for the selection of countries for the
UsageStatsCountry preference.

To test, apply the patch and restart services.

- Go to Administration -> System preferences and search for
  "UsageStatsCountry"
- The dropdown of countries should look correct.
- Select a country, save, and confirm that your choice is saved
  correctly.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>