Fix ExternalNav hitbox being smaller than what it looks like

Because <Anchor> was nested inside <Menu.Item>, the hitbox for the link element for both external and internal navigation was smaller than the size of the dropdown item. This merge request fixes that by exploiting how <Menu.Item> is a Mantine Polymorphic Component, which means that we can reduce one layer of nesting.

Some visuals (both highlighting the <a> element in rendered HTML):

Before	After