Add "App" tab to navigation

Make tab borders apply on a single side to allow for multiple tabs
Reduce font-size on smaller screens to ensure they all fit
Hide the github banner when the nav overlaps it
Make placeholder text smaller so more of it is visible

The placeholder rapidly overflows the input - especially on narrow
screens so let's reduce the font size.
Set line-height to 1em to vertically center everything
Revert "Add "App" tab to navigation"

This reverts commit 20a3508a2762dfd2e3f285aa106e85ab97d6975d.