Implement sorting of countries

[sawaymas]

Description:

• Let the user change the order countries are presented in.

Tasks:

• Add alphabetical sorting.
• Add sorting by population.
• Use a copy of the list when sorting.
• Connect the sorting choice to navigation and the country selector.
• Add a clear label.

Acceptance:

• Sorting changes the order of the countries.
• Navigation follows the sorted list.
• The original data is not mutated.
• The sorting control is accessible.