Feature: Nav bar

[jschuur]

• [ ] logo/site name separate and optionally hidden in mobile
• [x] separate NavItem component
• [x] underline (with offset) current entry
• [ ] load from array in main Nav component
• [ ] drop down for responsive layout (might be separate task)
• [ ] allow for shortened names in mobile view
• [ ] comment for optional link prefetching

[jschuur]

Partially implemented in a8684aeb2d47ba7a0199304b7eecdab5902b02ea