Add page components for nation subpages

[Frewacom]

The subnavigation links on the nation page currently does not have any defined page components. This means that they all return 404's.

Solution

• Create new nation directory in pages/admin/
• Move the current pages/admin/nation.tsx to pages/admin/nation/index.tsx
• Add new page components for each subpage (in pages/admin/nation/<subpage>.tsx
• Create a new layout for the nation page (based on the code in the current nation.tsx page)
• Reuse the newly created layout in all of the nation pages (you can see examples of this in components/layouts/admin/UserLayout.tsx and the users/[user_id]/*.tsx pages)