[torconnect] Add optgroups to country selector

Could we add a couple of optgroups to the country selector please, to help (vaguely) explain why certain countries are placed outside the full alphabetical list? e.g.:

Instead of popular, we could do "Frequently selected locations" if you prefer (I've gone with popular above since it's shorter).

Please ignore the fact the full list only includes South American countries here, it's just to demonstrate the idea :)